Unless a future R500 driver works without openfirmware requirements, the best R500 card available for the G5 is the Apple X1900GT.
The X1950 only work with Mac Pros (X86 machines).
And any R500 driver release is likely to be several months from now.
An R400 driver is likely to be released first (possibly quite soon).
In informal tests, I have found the X800XT slightly outperforms the X1900GT.
And Apple X800XT cards can be found at cheap prices on occasion.
Will there be support for R400 cards or are they being skipped like with OS4. I don't keep such a keen eye on what's going on with MorphOS so I was only aware of the R500 driver being in progress because of the demo of MorphOS on San460 at Pianeta.
I suppose later cards could be initialised from within the OS like with out 68k Amigas.