Outside flashing, there are four options for PCIe G5s. The factory options were the 6600, 7800GT, and Quadro FX4500. There was a retail Radeon X1900GT.
The Quadro is the "sexy" card that everyone wants, but for most folks the Radeon is the better card.
BTW-with regard to the above comment on overclocking cards-proceed with a bit of caution on single height Radeons(including the FireGL). They can put out a lot of heat. The stock cooler is fine for stock clock speeds, but rob a dual height cooler off a PC version of the card if you want to push things.