Sdashiki said:
No. I specifically mentioned the nVidia GeForce 6800GT, because I sucessfully flashed it with no hardware mods and can vouch for it.
On the ATI side, the best cards that are currently flashable are the Radeon 9800 family (9800 Pro + 9800XT) and the Radeon X800XT. HOWEVER, the PC version of the Radeon X800XT comes with a caveat. The Mac X800XT has two DVI ports; the PC version has one DVI and one VGA. The PC card can be successfully flashed to the Mac, but the DVI port will become non-functional...you are limited to the VGA port. This is why I did the 6800GT instead, since it can be flashed into a Mac card that retains both DVI ports' functionality. On the whole I'm an ATI fanboy and would hgave preferred the X800XT but I need a working DVI port. If you don't need the DVI, the X800XT is a great option.
A week or so ago, nVidia rolled out an AGP version of the GeForce 7800GS...which means that soon it may become an option to G5 owners with an AGP port who are looking at flashing a high-end card. Overall though, the 6800GT is a great performer - way better than the 9800 series.
But flashing is not for the lazy or impatient. I spent hours on the Strangedogs forum reading about how to do it and what possible problems might arise. In the event, I had no trouble but your mileage may vary. Read my thread about it by clicking on the 6800GT in my sig and go to Strangedogs for more specific info, as well as rom downloads.
There is one way to flash a fully working X800XT to the Mac: the ATI FireGL X3 workstation-class video card can be flashed with a Mac rom to make a perfect X800XT. However, the FireGL X3 is horribly expensive, at least a couple hundred bucks more than the real Mac Edition X800XT. So it isn't an option, really.