Check /etc/X11/xorg.conf
This is all you need in the Screen Section.
Your Identifier, Device, Monitor may be different.
You could post this part of xorg.conf
Code:
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 16
SubSection "Display"
Viewport 0 0
Depth 16
Modes "1024x768"
EndSubSection
EndSection