Somewhere or another out there, there is a relative listing for all the native and "flashable" G5 cards.
As I recall, the X850 is at the top of the list for cards that were available in a Mac version, although there are a few better performing cards that can be flashed.
Personally, I have a 6800 Ultra(ranked a notch or two below the X850 on the chart I've seen) and have been very happy with it. One of the big advantages of the 6800, at from what I've seen, is that it's probably one of the easiest high performance G5 cards to find(I think this may be due to the fact that it was a BTO option for the G5). Performance aside, one of the disadvantages of the 6800 vs. the x800/X850 is that the 6800 is a dual height card and the X800/850 standard height.
Also, depending on how you wish to use the card, I'll point out that the 6800 has dual DVI outs whereas the X800/850 has ADC and DVI out. ADC is relatively easy to turn into DVI with a $20-30 passive adapter, while turning DVI into ADC requires a $50-100 power brick adapter. There are flashed equivalents of the X800 that have dual DVI.