Check your /etc/X11/xorg.conf. Here is a sample of the lines you are interested at:
Code:
DefaultDepth 24
# DefaultDepth 32
# "1024x768" is also a conservative usable default resolution. If you
# have a better monitor, feel free to try resolutions such as
# "1152x864", "1280x1024", "1600x1200", and "1800x1400" (or whatever your
# card/monitor can produce)
Subsection "Display"
Depth 24
#Modes "1440x900" "1024x768" "800x600" "640x480"
Modes "1024x768" "800x600" "640x480"
#Modes "1600x1200"
EndSubsection
That basically will try to start your x-server with 24 bits dept and 1024x768 resolution. If that fails, it will try 800x600, then 640x480. Try adding your desired resolution under "Display" first. Let's say you want 1280x1024, then:
Code:
Subsection "Display"
Depth 24
#Modes "1440x900" "1024x768" "800x600" "640x480"
Modes "1280x1024" "1024x768" "800x600" "640x480"
#Modes "1600x1200"
EndSubsection
Remember to backup that file before changing anything. If the above does not work, you may need to install the appropriate drivers for your gfx card.
Regards!