Macs, for decades, have been inferior in the high performance graphics (games, high end games etc), and this has mainly been due to Apple configuring their machines with hard to find AGP slots, and propietry graphics gear. This is not the case any more. LittleBob currently has a Powermac G5 dual 2.3GHz, with an NVida 7800GTX card installed. A great card by anyones book, and I can tell you, it really does fly.
So how do you find these great cards?
Depending on price, there are a lot of good choices.
In the low end budget, you can reach out for: ATI Radeon Pro 9000 AGP (This is a 64MB card, and shipped in Powermac G4s, and typically has 1 X DVI/VGA slot, and 1 X ADC (apple display connector) onboard). Alternatively you can get a NVIdia Go5200 64mb card, which also has DVI and ADC. These ones shipped in early Powermac G5s, and will interface fine with a G4 and any new G5.
On the higher end of the scale (and for PowerMac G5s), you have a wonderful choice.
NVIdia 6800 Pro
ATI 9800 Pro
and the daddy: NVidia FX4500 QUADRO 512mb card.
All of these cards are made for PCs, and there are MAC versions as well. So when your looking for the card, ensure that it is AGP, NOT PCI!!
You can alo check on the companies websites or search something in google like: 'NVidia 6800 Pro Apple' or something to that effect, which should tell you what model numbers to look out for for your Apple.
So, thats all LittleBob can tell you right now. Good luck and HAPPY BIDDING for your new GFX card!