After upgrading my Debian system I also got xserver-xorg 7.4 (previously 7.3) and ended up with frequent crashes of the X server. It happens at different opportunities. It's easily reproducible here by taking "kaffeine" (the KDE media player) and resizing its window randomly for a few seconds. After that I get a crash. Backtrace:

(gdb) cont                                   
Continuing.                                  

Program received signal SIGSEGV, Segmentation fault.
0x080b7685 in xf86SIGIO (sig=29)                    
    at ../../../../../hw/xfree86/os-support/linux/../shared/sigio.c:110
110     ../../../../../hw/xfree86/os-support/linux/../shared/sigio.c: No such file or directory.                                                                
        in ../../../../../hw/xfree86/os-support/linux/../shared/sigio.c         
(gdb) bt f                                                                      
#0  0x080b7685 in xf86SIGIO (sig=29)                                            
    at ../../../../../hw/xfree86/os-support/linux/../shared/sigio.c:110         
        i = <value optimized out>                                               
        ready = {fds_bits = {50331648, -1075099752, 147026568, -1211464159,     
    54, 138069424, -1210498752, 2, 2, -1210503180, 2, 24, -1075100296,          
    -1211456107, -1210498752, 24, -1210498752, -1210503180, -1208962848, 2,     
    32, -1075100280, -1210503180, -1210498752, 138069424, -1075100248,          
    -1211465338, -1210498752, 138069424, 138069416, -1208962848, 0}}            
        to = {tv_sec = 0, tv_usec = 0}                                          
        save_errno = 0                                                          
        r = -1                                                                  
#1  <signal handler called>                                                     
No symbol table info available.                                                 
#2  0xb7cafece in memmove () from /lib/i686/cmov/libc.so.6                      
No symbol table info available.                                                 
#3  0x08134b25 in WriteToClient (who=0x8db1140, count=32, __buf=0x8da17a0)      
    at ../../os/io.c:826                                                        
        oc = (OsCommPtr) 0x8404310                                              
        oco = (ConnectionOutputPtr) 0x8f17b60                                   
        padBytes = 0                                                            
#4  0x08093cea in WriteEventsToClient (pClient=0x8db1140, count=1,              
    events=0x8da17a0) at ../../dix/events.c:5539                                
        eventCopy = {u = {u = {type = 240 '�', detail = 144 '\220',             
      sequenceNumber = 2295}, keyButtonPointer = {pad00 = 150442224, time = 1,  
      root = 136783064, event = 136180472, child = 3219867980, rootX = 18804,   
      rootY = -16405, eventX = 18792, eventY = -16405, state = 50974,           
      sameScreen = 21 '\025', pad1 = 8 '\b'}, enterLeave = {pad00 = 150442224,  
      time = 1, root = 136783064, event = 136180472, child = 3219867980,        
      rootX = 18804, rootY = -16405, eventX = 18792, eventY = -16405,           
      state = 50974, mode = 21 '\025', flags = 8 '\b'}, focus = {               
      pad00 = 150442224, window = 1, mode = 216 '�', pad1 = 36 '$',             
      pad2 = 39 '\'', pad3 = 8 '\b'}, expose = {pad00 = 150442224, window = 1,  
      x = 9432, y = 2087, width = 62200, height = 2077, count = 18764,          
      pad2 = 49131}, graphicsExposure = {pad00 = 150442224, drawable = 1,       
      x = 9432, y = 2087, width = 62200, height = 2077, minorEvent = 18764,     
      count = 49131, majorEvent = 116 't', pad1 = 73 'I', pad2 = 235 '�',       
      pad3 = 191 '�'}, noExposure = {pad00 = 150442224, drawable = 1,           
---Type <return> to continue, or q <return> to quit---
      minorEvent = 9432, majorEvent = 39 '\'', bpad = 8 '\b'}, visibility = {
      pad00 = 150442224, window = 1, state = 216 '�', pad1 = 36 '$',         
      pad2 = 39 '\'', pad3 = 8 '\b'}, createNotify = {pad00 = 150442224,     
      parent = 1, window = 136783064, x = -3336, y = 2077, width = 18764,    
      height = 49131, borderWidth = 18804, override = 235 '�',               
      bpad = 191 '�'}, destroyNotify = {pad00 = 150442224, event = 1,        
      window = 136783064}, unmapNotify = {pad00 = 150442224, event = 1,      
      window = 136783064, fromConfigure = 248 '�', pad1 = 242 '�',           
      pad2 = 29 '\035', pad3 = 8 '\b'}, mapNotify = {pad00 = 150442224,      
      event = 1, window = 136783064, override = 248 '�', pad1 = 242 '�',     
      pad2 = 29 '\035', pad3 = 8 '\b'}, mapRequest = {pad00 = 150442224,     
      parent = 1, window = 136783064}, reparent = {pad00 = 150442224,        
      event = 1, window = 136783064, parent = 136180472, x = 18764,          
      y = -16405, override = 116 't', pad1 = 73 'I', pad2 = 235 '�',         
      pad3 = 191 '�'}, configureNotify = {pad00 = 150442224, event = 1,      
      window = 136783064, aboveSibling = 136180472, x = 18764, y = -16405,   
      width = 18804, height = 49131, borderWidth = 18792, override = 235 '�', 
      bpad = 191 '�'}, configureRequest = {pad00 = 150442224, parent = 1,     
      window = 136783064, sibling = 136180472, x = 18764, y = -16405,         
      width = 18804, height = 49131, borderWidth = 18792, valueMask = 49131,  
      pad1 = 135644958}, gravity = {pad00 = 150442224, event = 1,             
      window = 136783064, x = -3336, y = 2077, pad1 = 3219867980,             
      pad2 = 3219868020, pad3 = 3219868008, pad4 = 135644958},                
    resizeRequest = {pad00 = 150442224, window = 1, width = 9432,             
      height = 2087}, circulate = {pad00 = 150442224, event = 1,              
      window = 136783064, parent = 136180472, place = 76 'L', pad1 = 73 'I',  
      pad2 = 235 '�', pad3 = 191 '�'}, property = {pad00 = 150442224,         
      window = 1, atom = 136783064, time = 136180472, state = 76 'L',         
      pad1 = 73 'I', pad2 = 49131}, selectionClear = {pad00 = 150442224,      
      time = 1, window = 136783064, atom = 136180472}, selectionRequest = {   
      pad00 = 150442224, time = 1, owner = 136783064, requestor = 136180472,  
      selection = 3219867980, target = 3219868020, property = 3219868008},    
    selectionNotify = {pad00 = 150442224, time = 1, requestor = 136783064,    
      selection = 136180472, target = 3219867980, property = 3219868020},     
    colormap = {pad00 = 150442224, window = 1, colormap = 136783064,          
      new = 248 '�', state = 242 '�', pad1 = 29 '\035', pad2 = 8 '\b'},       
    mappingNotify = {pad00 = 150442224, request = 1 '\001',                   
---Type <return> to continue, or q <return> to quit---                        
      firstKeyCode = 0 '\0', count = 0 '\0', pad1 = 0 '\0'}, clientMessage = {
      pad00 = 150442224, window = 1, u = {l = {type = 136783064,              
          longs0 = 136180472, longs1 = -1075099316, longs2 = -1075099276,     
          longs3 = -1075099288, longs4 = 135644958}, s = {type = 136783064,   
          shorts0 = -3336, shorts1 = 2077, shorts2 = 18764, shorts3 = -16405, 
          shorts4 = 18804, shorts5 = -16405, shorts6 = 18792,                 
          shorts7 = -16405, shorts8 = -14562, shorts9 = 2069}, b = {          
          type = 136783064, bytes = "��\035\bLI��tI��hI��\036�\025\b"}}}}}    
        eventTo = <value optimized out>                                       
        i = <value optimized out>                                             
        eventlength = 32                                                      
#5  0x08098828 in TryClientEvents (client=0x8db1140, dev=0x84ad2f8,           
    pEvents=0x8da17a0, count=1, mask=6479999, filter=32768, grab=0x0)         
    at ../../dix/events.c:1913                                                
        i = <value optimized out>                                             
        type = <value optimized out>                                          
#6  0x08099713 in DeliverEventsToWindow (pDev=0x84ad2f8, pWin=0x8ade8c0,      
    pEvents=0x8da17a0, count=1, filter=32768, grab=0x0, mskidx=0)             
    at ../../dix/events.c:1976                                                
        deliveries = <value optimized out>                                    
        nondeliveries = <value optimized out>                                 
        attempt = <value optimized out>                                       
        other = <value optimized out>                                         
        client = <value optimized out>                                        
        deliveryMask = <value optimized out>                                  
        type = 12                                                             
#7  0x08099e2b in DeliverEvents (pWin=0x8ade8c0, xE=0x8da17a0, count=1,       
    otherParent=0x0) at ../../dix/events.c:2421                               
        filter = 1                                                            
        deliveries = <value optimized out>                                    
#8  0x0811266c in miSendExposures (pWin=0x8ade8c0, pRgn=0x8f83d20, dx=216,    
    dy=355) at ../../mi/miexpose.c:446                                        
        pBox = (BoxPtr) 0x8f83d28                                             
        numRects = 1                                                          
        pEvent = (xEvent *) 0x8da17a0                                         
        pe = (xEvent *) 0x1                                                   
        i = 32                                                                
---Type <return> to continue, or q <return> to quit---                        
#9  0x081128bb in miWindowExposures (pWin=0x8ade8c0, prgn=0x8f83d20,          
    other_exposed=0x0) at ../../mi/miexpose.c:502                             
        expRec = {extents = {x1 = 3, y1 = 0, x2 = 322, y2 = 0}, data = 0x3}   
        exposures = (RegionPtr) 0x8f83d20                                     
#10 0x080d1d42 in xf86XVWindowExposures (pWin=0x8ade8c0, reg1=0x8f83d20,      
    reg2=0x0) at ../../../../hw/xfree86/common/xf86xv.c:1054                  
        pScreen = (ScreenPtr) 0x825ea38                                       
        WinPriv = <value optimized out>                                       
        pPrev = <value optimized out>                                         
        pPriv = <value optimized out>                                         
        AreasExposed = 0                                                      
#11 0x0812a7f4 in miHandleValidateExposures (pWin=0x8b70de0)                  
    at ../../mi/miwindow.c:245                                                
        pChild = (WindowPtr) 0x8ade8c0                                        
        val = (ValidatePtr) 0x8f83d20                                         
        WindowExposures = (                                                   
    WindowExposuresProcPtr) 0x80d1c90 <xf86XVWindowExposures>                 
#12 0x0812a418 in miSlideAndSizeWindow (pWin=0x8f2ed20, x=5, y=25,            
    w=<value optimized out>, h=<value optimized out>, pSib=0x0)               
    at ../../mi/miwindow.c:675                                                
        pParent = <value optimized out>                                       
        WasViewable = 1                                                       
        width = 486                                                           
        height = 352                                                          
        oldx = 5                                                              
        oldy = 25                                                             
        bw = <value optimized out>                                            
        oldpt = {x = 0, y = 0}                                                
        oldRegion = (RegionPtr) 0x8d89bf0                                     
        anyMarked = 1                                                         
        pScreen = (ScreenPtr) 0x825ea38                                       
        pFirstChange = (WindowPtr) 0x8f2ed20                                  
        pChild = (WindowPtr) 0x0                                              
        gravitate = {0x0, 0x8af1a58, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  
  0x0}                                                                        
        g = 11                                                                
        nx = 5                                                                
---Type <return> to continue, or q <return> to quit---                        
        ny = 25                                                               
        newx = <value optimized out>                                          
        newy = <value optimized out>                                          
        pRegion = (RegionPtr) 0x8390cc0                                       
        destClip = (RegionPtr) 0x8af1a58                                      
        oldWinClip = (RegionPtr) 0x8fb0bd0                                    
        borderVisible = (RegionPtr) 0x0                                       
        shrunk = <value optimized out>                                        
        moved = <value optimized out>                                         
        pLayerWin = (WindowPtr) 0x8f2ed20                                     
#13 0x08143af8 in compResizeWindow (pWin=0x8f2ed20, x=0, y=0, w=770, h=603,   
    pSib=0x0) at ../../composite/compwindow.c:406                             
        pScreen = (ScreenPtr) 0x825ea38                                       
#14 0x08078191 in ConfigureWindow (pWin=0x8f2ed20, mask=15, vlist=0x910f8f4,  
    client=0x8443300) at ../../dix/window.c:2403                              
        pSib = (WindowPtr) 0x0                                                
        pParent = (WindowPtr) 0x8b70de0                                       
        sibwid = 0                                                            
        index2 = <value optimized out>                                        
        tmask = <value optimized out>                                         
        pVlist = (XID *) 0x910f904                                            
        x = -2328                                                             
        y = <value optimized out>                                             
        w = <value optimized out>                                             
        h = <value optimized out>                                             
        bw = 0                                                                
        rc = <value optimized out>                                            
        action = 2                                                            
        smode = 0                                                             
        event = {u = {u = {type = 22 '\026', detail = 0 '\0',                 
      sequenceNumber = 27314}, keyButtonPointer = {pad00 = 1790050326,        
      time = 18875401, root = 52428808, event = 0, child = 0, rootX = 770,    
      rootY = 603, eventX = 0, eventY = 0, state = 8, sameScreen = 32 ' ',    
      pad1 = 3 '\003'}, enterLeave = {pad00 = 1790050326, time = 18875401,    
      root = 52428808, event = 0, child = 0, rootX = 770, rootY = 603,        
      eventX = 0, eventY = 0, state = 8, mode = 32 ' ', flags = 3 '\003'},    
    focus = {pad00 = 1790050326, window = 18875401, mode = 8 '\b',            
---Type <return> to continue, or q <return> to quit---                        
      pad1 = 0 '\0', pad2 = 32 ' ', pad3 = 3 '\003'}, expose = {              
      pad00 = 1790050326, window = 18875401, x = 8, y = 800, width = 0,       
      height = 0, count = 0, pad2 = 0}, graphicsExposure = {                  
      pad00 = 1790050326, drawable = 18875401, x = 8, y = 800, width = 0,     
      height = 0, minorEvent = 0, count = 0, majorEvent = 2 '\002',           
      pad1 = 3 '\003', pad2 = 91 '[', pad3 = 2 '\002'}, noExposure = {        
      pad00 = 1790050326, drawable = 18875401, minorEvent = 8,                
      majorEvent = 32 ' ', bpad = 3 '\003'}, visibility = {pad00 = 1790050326, 
      window = 18875401, state = 8 '\b', pad1 = 0 '\0', pad2 = 32 ' ',         
      pad3 = 3 '\003'}, createNotify = {pad00 = 1790050326, parent = 18875401, 
      window = 52428808, x = 0, y = 0, width = 0, height = 0,                  
      borderWidth = 770, override = 91 '[', bpad = 2 '\002'}, destroyNotify = {
      pad00 = 1790050326, event = 18875401, window = 52428808}, unmapNotify = {
      pad00 = 1790050326, event = 18875401, window = 52428808,                 
      fromConfigure = 0 '\0', pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'},    
    mapNotify = {pad00 = 1790050326, event = 18875401, window = 52428808,      
      override = 0 '\0', pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'},         
    mapRequest = {pad00 = 1790050326, parent = 18875401, window = 52428808},   
    reparent = {pad00 = 1790050326, event = 18875401, window = 52428808,       
      parent = 0, x = 0, y = 0, override = 2 '\002', pad1 = 3 '\003',          
      pad2 = 91 '[', pad3 = 2 '\002'}, configureNotify = {pad00 = 1790050326,  
      event = 18875401, window = 52428808, aboveSibling = 0, x = 0, y = 0,     
      width = 770, height = 603, borderWidth = 0, override = 0 '\0',           
      bpad = 0 '\0'}, configureRequest = {pad00 = 1790050326,                  
      parent = 18875401, window = 52428808, sibling = 0, x = 0, y = 0,         
      width = 770, height = 603, borderWidth = 0, valueMask = 0,               
      pad1 = 52428808}, gravity = {pad00 = 1790050326, event = 18875401,       
      window = 52428808, x = 0, y = 0, pad1 = 0, pad2 = 39518978, pad3 = 0,    
      pad4 = 52428808}, resizeRequest = {pad00 = 1790050326,                   
      window = 18875401, width = 8, height = 800}, circulate = {               
      pad00 = 1790050326, event = 18875401, window = 52428808, parent = 0,     
      place = 0 '\0', pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'},            
    property = {pad00 = 1790050326, window = 18875401, atom = 52428808,
      time = 0, state = 0 '\0', pad1 = 0 '\0', pad2 = 0}, selectionClear = {
      pad00 = 1790050326, time = 18875401, window = 52428808, atom = 0},
    selectionRequest = {pad00 = 1790050326, time = 18875401, owner = 52428808,
      requestor = 0, selection = 0, target = 39518978, property = 0},
---Type <return> to continue, or q <return> to quit---
    selectionNotify = {pad00 = 1790050326, time = 18875401,
      requestor = 52428808, selection = 0, target = 0, property = 39518978},
    colormap = {pad00 = 1790050326, window = 18875401, colormap = 52428808,
      new = 0 '\0', state = 0 '\0', pad1 = 0 '\0', pad2 = 0 '\0'},
    mappingNotify = {pad00 = 1790050326, request = 9 '\t',
      firstKeyCode = 4 '\004', count = 32 ' ', pad1 = 1 '\001'},
    clientMessage = {pad00 = 1790050326, window = 18875401, u = {l = {
          type = 52428808, longs0 = 0, longs1 = 0, longs2 = 39518978,
          longs3 = 0, longs4 = 52428808}, s = {type = 52428808, shorts0 = 0,
          shorts1 = 0, shorts2 = 0, shorts3 = 0, shorts4 = 770, shorts5 = 603,
          shorts6 = 0, shorts7 = 0, shorts8 = 8, shorts9 = 800}, b = {
          type = 52428808,
          bytes = "\000\000\000\000\000\000\000\000\002\003[\002\000\000\000\000\b\000 \003"}}}}}
#15 0x0808bd62 in ProcConfigureWindow (client=0x8443300)
    at ../../dix/dispatch.c:741
        pWin = (WindowPtr) 0x8f2ed20
        result = <value optimized out>
        len = 4
        rc = <value optimized out>
#16 0x0808c7af in Dispatch () at ../../dix/dispatch.c:437
        result = <value optimized out>
        client = (ClientPtr) 0x8443300
        nready = 0
        start_tick = 1280
#17 0x0807198d in main (argc=9, argv=0xbfeb4ee4, envp=Cannot access memory at address 0x28
) at ../../dix/main.c:397
        i = <value optimized out>
        alwaysCheckForInput = {0, 1}
(gdb)

-----------------------------------

My /etc/X11/xorg.conf:

Section "ServerLayout"
    Identifier     "Default Layout"
    Screen         "Default Screen" 0 0
    InputDevice    "Generic Keyboard"  
    InputDevice    "Configured Mouse"  
EndSection                             

Section "Files"
    FontPath        "unix/:7100"                                          
        FontPath        "/usr/share/fonts/X11/misc"                       
    FontPath        "/usr/lib/X11/fonts/misc"                             
        FontPath        "/usr/share/fonts/X11/cyrillic"                   
    FontPath        "/usr/lib/X11/fonts/cyrillic"                         
    FontPath        "/usr/lib/X11/fonts/100dpi/:unscaled"                 
    FontPath        "/usr/lib/X11/fonts/75dpi/:unscaled"                  
        FontPath        "/usr/share/fonts/X11/Type1"                      
    FontPath        "/usr/lib/X11/fonts/Type1"                            
        FontPath        "/usr/share/fonts/X11/CID"                        
    FontPath        "/usr/lib/X11/fonts/CID"                              
        FontPath        "/usr/share/fonts/X11/100dpi"                     
    FontPath        "/usr/lib/X11/fonts/100dpi"                           
        FontPath        "/usr/share/fonts/X11/75dpi"                      
    FontPath        "/usr/lib/X11/fonts/75dpi"                            
EndSection                                                                

Section "Module"
    Load           "bitmap"
    Load           "dbe"   
    Load           "ddc"   
        Load    "dri"      
    Load           "extmod"
    Load           "freetype"
    Load           "glx"     
    Load           "int10"   
    Load           "record"  
    Load           "vbe"     
EndSection                   

Section "InputDevice"
    Identifier     "Generic Keyboard"
    Driver         "kbd"             
    Option         "CoreKeyboard"    
    Option         "XkbRules" "xorg" 
    Option         "XkbModel" "pc105"
    Option         "XkbLayout" "de"
    Option         "XkbVariant" "nodeadkeys"
EndSection

Section "InputDevice"
    Identifier     "Configured Mouse"
    Driver         "mouse"
    Option         "CorePointer"
    Option         "Device" "/dev/input/mice"
    Option         "Protocol" "ImPS/2"
    Option         "Emulate3Buttons" "false"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "Monitor"
    Identifier     "Standardbildschirm"
    HorizSync       30.0 - 65.0
    VertRefresh     50.0 - 75.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Standardgrafikkarte"
    Driver         "nvidia"
EndSection

Section "Screen"
    Identifier     "Default Screen"
    Device         "Standardgrafikkarte"
    Monitor        "Standardbildschirm"
       Option   "AddARGBGLXVisuals"   "true"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
        Modes      "1280x1024"
    EndSubSection
EndSection

Section "Extensions"
        Option "Composite" "true"
EndSection

-------------------------

I also encountered similar crashes (didn't examine the backtrace then though) with an empty xorg.conf. The problem occured both with the free "nv" driver as well as with the nvidia.com non-free driver "nvidia".

My graphics hardware (lspci):

01:00.0 VGA compatible controller: nVidia Corporation G73 [GeForce 7600 GS] (rev a2)

I downgraded Xorg to 7.3 and the crashes seem to have gone away.

 Christoph