You can go into your X config file and select some settings like 640x480 as well as 8 bit color depth.
usually /etc/X11/XF86Config:
Section "Screen"
Identifier "Screen0"
Device "ATI Radeon VE"
Monitor "Monitor0"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1600x1200" "1400x1050" "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
EndSection
Change your depth to 8 and put 640x480 the only 'Mode'. I would comment out the default config and create your own screen section so you dont mess up the original (like it matters, doesnt work anyways

)