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.
My PowerMac G5 has X1950Xt 256 MB and it works perfectly (OF+MacosX)
