I have just tried booting Open Firmware, and the GTX showed the firmware while the AMD didn't.
You will need Apple -spesific bios and since there is none, it will not work.
You will need Apple -spesific bios and since there is none, it will not work. That's the whole point when (for example) flashing Radeon 9800 from PC to Mac, replacing the PC bios with Mac bios. However, there was atleast one special edition card which worked in both PC and Mac without any tricks, I think it was from the Radeon 9000 -series... but anyways, you need correct type of bios.
EDIT: Yup, this was it:
http://www.ati.com/products/radeon9600/radeon9600propcmac/
Why would you want a card like the 7850 in your powermac? Lets say there were drivers and everything to support your card fully in the powermac. And drivers to fully support 32 gigs of ram.
What on earth program built in the day and age of the g5 will really use up quad cores much less all the power of a great modern card like the 7850 and 32 gigs of ram?