Yes- this works just fine- you'll need:

find the bios that matches to your 9600 (preferably one from ATI) and make sure it's not an AIW card

matching the PRO / XT, etc ratings helps for memory timings-
also, match amount of ram or you might have issues.

once you have the rom and the atiflash, you should be able to (on a pc with a PCI graphics card):

set the bios to boot from the pci as primary.

put in the agp card, boot the system into dos.

run the ati flash and make a backup of the old 9600 g5 bios
run the ati flash and forcably flash the new bios to the card.

shutdown and test.

If the card has physical and electrical differences, before sticking it in the pc, you may need to do this mod, or one similar.

I have never done this on a G5 card, however, my flatmate has done the 5200 mod, and I have converted 2 9800 Pro Retail cards to PC use after I stopped using them in my macs, using the procedure I outlined above.

Good luck, and let us know how it goes!