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.

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My quad G5/2.5 (8GB RAM, etc) has a 7800GT. Money well spent. I intend to get a lot of wear out of this machine; it's served me well for 3 1/2 years already.

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As long as we have some browsers still being developed, then PPC will be able to hold on for a good long time. Safari 4 is still compatible with old iMac G3s so it would be surprising to see them just hack off all PPCs in one fell swoop.

It looks like Flash Player 10.1 will support PPC eventually so that's good.

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Well, the end is near. Adobe AIR 2.0 won't support PPC anymore, for example, so I confidently expect Flash 11 to be x86 only. Myself, I'm gearing up to running off my own Caminos or Firefoxen in the same way I run off Classillas in the future, both because I use Tiger, and because this G5 -- one of the last, mind you, I bought it in 8/06 -- is now 3 1/2 years old, and frankly it's a cold Intel world out there. :-/