If you have an AGP slot, try to buy a used (or new) PC AGB card and flash it.

Several sites contain information:

http://www.ehmac.ca/anything-mac/27249-flashing-vga-cards-mac.html

http://www.cybercoment.com/macgeforce.htm

http://forums.mactalk.com.au/33/28635-flashing-geforce-video-card-mac.html

http://xenomorph.net/apple/flash-pc-video-cards-for-use-in-a-mac/

http://www.kuro5hin.org/story/2006/6/3/234045/4651

Just use google, the second link is good.

G4 cube users flash graphic cards for their machines.