With work on the drivers for X1xx0 series cards under way it shouldn't matter much. G5 + Radeon X1950 will be pretty awesome. Look forward to seeing that when it happens.
Is there any word from the MorphOS team on what direction they're taking with regard to OpenGL?
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.