I have the X800. I purchased it after my G5 so I suppose I could have had either. The main reasons I chose the X800 were:
- Equal to and in some cases better performance than the 6800
- significant lower cost
- over clocking
- uses one slot (6800 uses one, but covers two)
- more frequent updates from ATI
One think to keep in mind is that you can only run one 30" display with the x800 where you can have two with the 6800.
Here is the link to the benchmark test between the x800 and the 6800.
http://www.barefeats.com/radx800.html
HTH