Just note the following if you go the Radeon 9800 Pro route.
There are two types of 9800 Pro when it comes to Mac, three if you count a flashed PC card.
The first was a BTO option for the G5. That card will work in a G4 but you have to tape the pins. It's easy to tell that it's for a G5 because the card is blue.
Shortly after ATI shipped those cards for Apple G5s, ATI released a retail version of the 9800 Pro. That card is red and it's compatible with the G4 and the G5 and you do not need to tape pins.
The one flashed 9800 Pro that I used to have does not work in a Quicksilver with taped pins.
Lastly, there is a rare Special Edition of the 9800 Pro that is compatible with the G4/G5. The difference is that it's a 256mb card. All other 9800 Pros have 128mb.