You're, as always, better off to run xf86config (as root) and make the settings there. I only run 1024 X 768, so, I change all the color depth display modes so that they can only run 1024 X 768.
Where is this? You're going to get to a certain menu in xf86config where it shows all the modes. You will have a few options and at the bottom you'll see the option that says "The modes are ok." Before you use that one change your modes to 1024 X 768.
Or even better, hand edit the XF86Config-4 file in /etc/X11. Look for display section and make the change.
Extremely easy but brief: http://www.scorpioncity.com/linux.html#xqui