Better aftermarket video card for G5 Quad

I have a G5 quad with the standard NVidia 6600 Card in it. I wanted to
upgrade it to a 7800GT or better. But the nvidia web site (And the
apple one) seems to state that that can only be done at order time.

Is this true? Which aftermarket cards are compatible with G5 quads, I
know it takes PCI-E, does it take any NVidia or ATI PCI-E card?

Since I can not seem to find any specific device drivers anywhere I
wonder if I can get external cards that will work right of the bat?

Thanks,

Hank.