In /etc/X11/XF86Config (or XF86Config-4), you'll find the following lines:
Code:
Section "Screen"
Identifier "Screen 1"
Device "S3 Savage4"
Monitor "Samtron 96P"
DefaultDepth 16
Subsection "Display"
Depth 8
Modes "640x480" "800x600" "1024x768" "1280x1024" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
EndSection
This is where you can edit screen resolution and color depth (DefaultDepth). Resolutions are used from left to right; if the first one doesn't work, the next one is tried. You can also change the resolution when running X by pressing CTRL + ALT + numpad - or CTRL + ALT + numpad +, it cycles through those resolutions.