howagood,

you'll want to look at /etc/X11/XF86Config-4

here is part of my file, look for this part in your XF86Config-4 file:

Section "Screen"
Identifier "Default Screen"
Device "Generic Video Card"
Monitor "Generic Monitor"
DefaultDepth 24
(snip)...
SubSection "Display"
Depth 24
Modes "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
(snip)...

in the section that corresponds to your default display depth (as root) edit the file and either make a copy of the Modes line and comment the original out or just edit it so it something like:
...
Depth 24
#Modes "1152x864" "1024x768" "800x600" "640x480"
Modes "800x600" "640x480"
...

then save the file and restart your X server. Be careful because if you mess up you won't be able to run X, it's probably a good idea to make a backup copy of your existing XF86Config-4 file

HTH

rock