Anarchy,
Back in the day I had a PC 6800 GT (I actually had 2) running in my G4 digital Audio. One of them is now happily running in a G5 (w/ an AGP slot. PCI-e is a different story) There are lots of cards that can be cross-flashed to Mac but it is not a task for the faint of heart. There were some awesome sites just for this type of project but they have fallen by the wayside. One current site that still has some info on the how-to of flashing and editing ROMs, tapping pins (this involved covering pins on the GPU that cotacted the pins Apple used for power with the ADC connector), swapping ROM chips, etc. is
http://www.aqua-mac.com I would read his 6800GT mac flash link. There was also sucess with 7800 cards but only if you had the right core aka G70 not G71. Another invaluable resource is
http://themacelite.wikidot.com/. This was/is THE resource for ROMs, etc.! As a disclaimer please note that many a card died a horrible death learning how to do this and borking a ROM can, in some cases, ONLY be corrected by soldering on a new ROM chip.
Nik
P.S. Jeez how time flies! I remember when the flashed GPUs were bleeding edge.