Voodoo AGP cards definitely won't work in a G5, the G5 with its 8x slot doesn't support the voltage they run at.
The Voodoo4 or 5 PCI cards would be an interesting option, but due to a driver that caters to the Peg2, these cards do not produce the correct colors in a Mac.

I have a Voodoo3 2000 PCI on hand that I know works in a G4, but I have promised that to a Swedish friend (and I am afraid of damaging it just for the sake of finding out if it works).
I also have a Rage128 PCI card, but it does not current have an Apple rom.
And the Rage128 driver is un-accelerated.

It not a terribly important issue, but I did have dual monitors set up in one of my G4s.

I will probably flash a spare Rage128 this week in order to test that out (I can spare that).

That would allow me (theoretically) to run the X1950Pro AGP card I have on order, and still boot up OSX when I feel like it.