if you are using XFree86 you need to edit your /etc/X11/XF86Config-4 file and look for the res settings. # out the higher settings. it sounds like your x-config is trying to run at a higher setting then your vid card or monitor can handle.
Code:
Section "Screen"
Identifier "Default Screen"
Device "ATI Technologies, Inc. Radeon Mobility M7 LW [Radeon Mobility 7500]"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1024x768" "800x600" "640x480"
EndSubSection
you probably have lines in there with as high as 1600x1200, if so you need to keep removing the line until you get to a working desktop, then you can start troubleshooting as to why you have to run at the lower settings.
in the above example my laptop will run no higher then 1024x768 thus will run at 800x600 and 640x480. if i try to run at say 1280x1024 i would get the same error you are getting. my laptop just can not run at those higher resolutions.