Created attachment 16197 [details]
the xorg.conf

the xorg configuration for xrandr 1.2 capable drivers has changed since you may have multiple monitors attached.  You'll need to specify a preferred mode per-monitor now.  Add the following to your monitor section to choose 1600x1200:

Option "PerferredMode" "1600x1200"

See this page for more info about xrandr 1.2:
http://www.intellinuxgraphics.org/dualhead.html

With this option the server hangs with a black screen. Seems that the radeon driver's check of the monitor's internals fails while the server's check doesn't (see attachment).

I don't understand the behavior so or so. The overview you linked to shows that the "Modes" argument is still valid and dominant. The argument in the monitor's section only chooses a preferred mode of the modes listed with the "Modes" argument. In both cases, this was 1600x1200. However, the driver fell back to 800x600, which wasn't set at all. Shouldn't the server turn down in this case? I'd understand this choice if the driver found higher modes but the refresh rate was too slow. However, the refresh rate for 1280x1024 is the same.

Created attachment 16223 [details]
the new xorg-log

(In reply to comment #3)
> With this option the server hangs with a black screen. Seems that the radeon
> driver's check of the monitor's internals fails while the server's check
> doesn't (see attachment).

looks like it's hitting an infinite loop in the server, probably because you have more than one 1600x1200 mode for that output.  try adding a unique modeline to your monitor section and specifying that instead.
Modeline "1600x1200_75.00"  204.75  1600 1720 1888 2176  1200 1203 1207 1255 -hsync +vsync
Option "PreferredMode" "1600x1200_75.00"

> 
> I don't understand the behavior so or so. The overview you linked to shows that
> the "Modes" argument is still valid and dominant. The argument in the monitor's
> section only chooses a preferred mode of the modes listed with the "Modes"
> argument. In both cases, this was 1600x1200. However, the driver fell back to
> 800x600, which wasn't set at all. Shouldn't the server turn down in this case?
> I'd understand this choice if the driver found higher modes but the refresh
> rate was too slow. However, the refresh rate for 1280x1024 is the same.
> 

the modes listed in the screen section are irrelevant for randr 1.2 drivers.  The default mode is based on the preferred mode in your monitor's edid unless you override it.
The driver is coming up in 1024x768 according to your log:
(II) RADEON(0): Output VGA-0 using initial mode 1024x768
this makes sense as this is the first detailed timing block from your edid:
(II) RADEON(0): Supported additional Video Mode:
(II) RADEON(0): clock: 94.5 MHz   Image Size:  306 x 230 mm
(II) RADEON(0): h_active: 1024  h_sync: 1072  h_sync_end 1168 h_blank_end 1376 h_border: 0
(II) RADEON(0): v_active: 768  v_sync: 769  v_sync_end 772 v_blanking: 808 v_border: 0

The ModLine provides a very crisp screen. Many Thanks! It's pushed to the left for about 50px, though. However, don't forget to fix the infinite loop.