Peter, one of the intelligent readers of this blog, wrote in with a nice money saving workaround to upgrade your video card to the best available option.
He suggests flashing a compatible FX4500.
In most benchmarks that I’ve seen, the FX4500 does edge the 7800GT in some tests. It was really designed to be more of a workstation card as opposed to a general usage or gaming card. You will probably see more benefits if you are rendering video or something of that nature, but as prices have fallen, it is cheaper to max out your G5 by grabbing one from eBay. Mac compatible ones, which may be just flashed cards, run around $100, but PC versions are available for $40-50.
Here’s an alternate route – get a PC version of this card and flash it. Peter tells us how:
It is trivial to flash a PC version of the NVIDIA Quadro FX4500 with the Mac firmware. These cards can be obtained on eBay for ~$30 and are much cheaper than the “Mac” versions, that typically cost around $100+. The only important thing is to make sure that the card you get has RAM on both sides of the card. Newest versions of the card only have RAM on one side of the card (the fan & cooler side) and will not work with the G5 (but they will work on the Mac Pro with the correct firmware).
This card has RAM on the back side of the board and is good for flashing with the G5 firmware:
This card does not have RAM on the back side and will not work with the G5 firmware:
You can find the ROM and utilities over at the Mac Elite wiki site.
This info has me tempted to try it on my own.