ati driver 6.7.194 makes GNOME using wrong resolution due to bogus monitor detection.
There's both a GNOME bug which consider the setting of the wrong monitor but also, in the first place, an ati driver bug which detects not present monitors (so root cause of bug #11781 and the like) was *not* fixed.

# with 6.6.3 driver:
$ xrandr
Screen 0: minimum 400 x 300, current 1024 x 768, maximum 1024 x 768
default connected 1024x768+0+0 0mm x 0mm
   1024x768       60.0* 
   800x600        60.0     56.0  
   640x480        60.0  
   400x300        60.0  

# with 6.7.194 driver:
$ xrandr
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 1152 x 1152
VGA-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 320mm x 240mm
   1024x768       60.0*+
   1152x768       54.8  
   800x600        60.3     56.2  
   640x480        66.7     69.6     60.0  
LVDS connected 800x600+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   800x600        60.3*    60.3  
   640x480        59.9  
None disconnected (normal left inverted right x axis y axis)

GNOME wrongly selects "800x600        60.3*" instead of "1024x768       60.0*+" but there's actually only a CRT monitor plugged.

This bug exists since at least 6.7.191 but I think it was already there in 6.6.19x