You need to determine whether you have a Yikes (PCI) or Sawtooth (AGP) G4. I belive the Sawtooth's AGP socket is brown, while the Yikes has only white PCI slots...
Anyway, once you've determined that, make sure you have 512MB-1GB RAM minimum to take full advantage of a better-than-stock video card setup.
A Yikes G4 can only handle PCI cards, so your only new option is a
Radeon 9200 Mac Edition. It is a good performance match for a stock Yikes, but it is 5 generations old and $130.
You can also buy a GeForce FX5200 or 5500 PCI card for the PC and flash a modified Mac ROM into it. It's considerably more modern in terms of feature set and faster too, but the flashing process is not necessarily for beginners...
If you have a Sawtooth, you have a lot more options in terms of flashing cards, but really only one retail option - the
Radeon 9800 Pro Mac Edition. This four generation-old warhorse retails for a breathtaking $250, but will provide all the performance a 400MHz G4 can handle and then some. They can also be flashed pretty easily with a Mac ROM and PC versions can be had for much less than $50 on ebay.
Beware people selling flashed cards on ebay - they may or may not be shafting you by misrepresenting their flashed card as a "real" OEM Apple card. Don't pay OEM prices for a PC card with a hacked Mac ROM.
There are other options as well, check out the
Strangedogs video card flashing forum and its accompanying
wiki.