I've never flashed back to PC, but I figure it's doable. This is what you'd use:
http://thomas.perrier.name/graphiccelerator.html
You'll also need a PC ROM for the card, you can probably find one on ATI's site. When it's done (provided it works) you'll need a second GPU installed because the card will immediately stop working in OSX. I suggest googling around for procedures/examples to be sure it works. Google for "strangedogs" for info on Mac card flashing/ROMs, most of their stuff is geared towards PC to Mac, but they might have some instrux for going the other way, I can't remember...
You might also want to consider selling the Mac card and buying a PC version (or even a higher grade card); using ebay you can probably make a few dollars as the Mac versions tend to sell for more.