You're looking in the right place. Down the bottom of the xorg.conf file are some lines like this:
Section "Screen"
Identifier "Default Screen"
Device "nVidia Corporation NV44 [Quadro NVS 285]"
Monitor "LCD"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
While you may have a different entry for the device and monitor, it is reasonable to add some entries in the "modes" lines for other resolutions, like those in the example above. I have had the same problems with RH in the past, and sorted it very successfully with the above fix.
The other thing that is worth doing is going to
http://www.nvidia.com/object/linux_d...173.14.12.html and getting the driver...generally manufacturer's drivers play well, and are worth trying
HTH