Not an expert on all those number-salad GPUs/GFX-cards but from the cards listed it seems there is a gap between R400 based GPUs (with 3D) and R600 based GPUs (2D only). Your card seems to be R500 based.....


As for running 2 cards, that works for sure. Just not when both cards are served by the same driver and since "Radeon" cover anything from the ancient Radeon7000 to the semi-current HD cards you are out of luck.

On a G4 PowerMac one can run a PCI Voodoo for a 2nd display, no idea if that would work in the X5000.