The /etc/X11/XF86Config-4 file is the one you want to modify. Towards the bottom, there ought to be a section that looks like this:
Code:
Section "Screen"
Identifier "screen1"
Device "device1"
Monitor "monitor1"
DefaultColorDepth 24
The DefaultColorDepth line specifies which color depth X will start in. Below that, there will be several portions like this:
Code:
Subsection "Display"
Depth 8
Modes "800x600" ...
EndSubSection
Those define the color depths and resolutions you'll have available at those depths. Unless you know what you're doing, don't mess with these too much; the resolution supported at a specific depth depends on your graphics card and monitor, so the values that are already there should be appropriate for your hardware.
