> So you are using two GFX Cards in your G5? One Nvidia Mac and a Radeon PC?

He's apparently not the only one who does, see comments #3 and #6 for instance.

> does somebody know what is the latest Radeon supported by OSX 10.5.8 PPC?

X1900 GT Mac edition. Other X19x0 cards may be supported when flashed with the PPC ROM from the X1900 GT Mac edition.