You need to purchase a video card that is made specifically for the Mac. There is a way to flash some PC GeForce cards' ROM but you will have to look on xlr8yourmac.com to find the site that tells you how to do this.
Ati is one of the only manufacturers that make aftermarket AGP cards for the Mac. Your choices depend on which model you have. If you have an older G4 Sawtooth model, you only have a 2x AGP slot and won't be able to use an AGP 8x video card (like those that are in G5's). You can find Ati Radeon cards on eBay for around $40 but they are not much faster than the Rage128 that is in a lot of older G4 PowerMacs. There are Radeon 9000 cards available but at ~$150, I don't think the price is justified. Your other options are Radeon 8500, which can be found on eBay for ~$140 which may be the best you will be able to get for an older PM. The problem is that when you start adding up all the upgrades that would be required to get your older PM up to the same speed as new ones, you could just buy an eMac, which will have a faster memory bus, faster hard drive bus, and faster video card.
