Umm... yeah, no. Price difference between Mac versions and PC versions is eye watering (for someone on my budget).Pick up a Mac 4870 and call it a day, 5xxx and 6xxx cards should work OoB too iirc
PC 4870 from around $35 shipped
Mac 4870 from around $150 shipped
If I want to upgrade, it has to be a flashed PC card, or it's just too expensive (unless I'm mind blowingly lucky).
It has an X1900XT in it (I had a PC X1900XTX to flash for the G5) and there's a gainward 7950GT in a box around here someplace that I bought in error for the G5.