The best native video card you can get for it is the 7800GT 256MB. I believe this was only ever sold as a BTO option on a new dual core G5 so they're quite hard to find.

You can flash certain models of PC 7800 or 7900 with a Mac ROM, and that's usually the cheapest option.

I have also heard that X1800s will work in a G5 but I'm not 100% sure on that one.