1200x800 is not a standard resolution---it should be maybe 1280x800 (16:10) or 1200x900 (4:3)
The most straighforward way is to hand-edit /etc//X11/xorg.conf. Make sure the Horiz and vertical entries in the "Monitor" section are correct (or just comment them out by adding "#" at the beginning of the line)
In the screen section, make an entry for the highest resolution you want to use--delete all the others. Here is a typical (simplified) Screen section:
Code:
Section "Screen"
Identifier "Screen 1"
Device "My Video Card"
Monitor "My Monitor"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1920x1200"
ViewPort 0 0
EndSubsection
EndSection
You can also run "xorgconfig"