The pcie ATI x1900gt is latest card supported by macosx 10.5.8 for powermacs.
Under morphos 3.12 it can be used in combination with native nvidia 6600 pcie card, using an pcie 8x free slot.

It can be also flashed with a powermac rom on a pc under msdos and atiflash tool.
Be careful because there are two version of x1900gt one with extra power connector on the top/left

radeon-x1900-gt.c173#gallery-1

and another with power connector on the middle/left

b7776ccd86677e084c73066d56234a62_XL.jpg

in my experience only the first can be modded in an original powermac ati x1900gt and so use only one video card instead of two.

I don't know if you can flash an ATI 1900xt card into a genuine powermac x1900gt, if someone has tested it please let us know.

About powermacs g5 quad I have to say it's a big beast and LCS issues are a big mistake.
On my G5 quad I had to rebuild the whole cooling system using special liquid cooled nordbrige heatsink and a special liquid metal thermal compound.
Now I have the fastest amiga ever seen..