Try booting into runlevel 3. in a terminal, su to root if not logged in as root, and type "/sbin/init 3". once out of X and back to the command line type "more /etc/X11/XF86Config-4" Look at the section that says screen, you should have a mode line there something like this: Modes "1024x768" "640x480", if its there and you cant use anything higher then 640x480 then there might be a prob with your driver. If it says "640x480" then try changing it to 800x600 or 1024x768. You can edit the file using vi. example: vi /etc/X11/XF86Config-4.
once the file is open in vi, you hit i to insert and your arrow keys to navigate. If you mess up and get to lost then hit esc then shift+: and type q! enter, that will quit without making changes, if everything looks good then instead of q! do a wq! which will save and quit. then restart x with startx
make sure your video card can support your default color depth also, ie. 8, 16, 24
heres an example of what your config file might look like: http://www.xiph.org/~jack/ibook/XF86Config-4