The difference between a Mac AGP graphics card and a PC AGP graphics card, besides the price, is just the software in the flash ROM on the card--the PC version contains Intel driver software, the Mac version contains Mac driver software. Since the video cards are firmware flashable, it's possible to buy a PC AGP video card, get a copy of the Mac firmware as a ROM file, flash it onto the card, and end up with a Mac card.
Power Mac G5 systems come standard with an nVidia GeForce 5200 card, and you can buy the PC version of this card for about $80, so we bought a PNY GeForce card from CompUSA and set about installing the Mac firmware on it.
Doing this is A Big Pain In the Ass.
Problem 1; The nVidia flash ROM utility only runs from a floppy disk and only in DOS mode. Why PC users put up with crap like that, in this day and age, is beyond me. (The Mac version of the nVidia flash ROM utility works in the GUI just like any other program, but can't be used because if you put a PC card in the Mac, you have no video until you run the flasher, and if you have no video, you can't...err, run the flasher.)
Problem 2: Getting the Mac ROM image is A Big Pain In the Ass. It's hard to find the ROM image because...well, I don't know why. Apple goes after people who distribute the ROM image files, which is kind of weird because Apple does not sell video cards. If Apple did sell video cards, it'd make more sense, because Apple would have a vested interest in keeping people from buying PC cards and flashing them, but as Apple does not sell video cards except as included with Mac systems, this makes little financial sense.
And finally, after five hours of formatting floppies, installing the nVidia flasher utility, installing a PCI graphics card in the PC so I can flash the AGP card, and searching online for a kind soul with the Mac graphics firmware image, we come to Problem 3...
...which is that the GeForce card I bought is a PNY branded card.
Okay, just for the record: Never, ever, ever buy a PNY-branded video card.
A PNY-branded GeForce card is cheaper than, say, a Creative-branded GeForce card, and as it turns out, there's a reason why. PNY, you see, uses different (cheaper) RAM and a different (cheaper) RAM controller on their video cards.
So, the firmware that turns a PC GeForce card into a Mac GeForce card works in any GeForce card except PNY-branded cards. PNY-branded video cards, when flashed with Mac firmware, won't work, because the cheaper, slower RAM in the PNY-branded card is not accounted for in the Mac firmware.
Suck, suck, suck.
The good news: i kept a copy of the original PNY firmware, so I flashed it back and the card is good as new. We're taking it back tomorrow. The bad news: Shelly still has to play World of Warcraft on her crap Sony PC, which is no end of aggrevating, and I'll never have those five hours of my life back again.