I’m popping back up to update the site and point to this interesting thread on the MacRumors forum about an option for an improved video card on the cheap for our G5s.
The Radeon X1900 Mac Edition was an expensive aftermarket card in its day and is near the top of the performance level for Power Mac G5s. The FX4500 is probably slightly better as a workstation card, but the X1900 holds its own against the 7800GT. If the method below works for you, it will save you a bundle of cash and give you another option for your aging PowerPC machine.
Let me know if you have any luck.