The reviews have them neck and neck, but I think with driver updates and ps3 support, the 6800 will pull ahead over time. Most reviews also put a lot of the benchmarking back on CPU performance which has got me thinking AMD 64 bit upgrade. Thye both seem to be vaporware for the most part. I ordered a 6800 ultra 2 weeks ago from PNY and it's still back ordered. eVGA's site says no stock until the first of July, and NewEgg must get 2 or 3 in at time because they'll pop up briefly, albeit severely overpriced, but mostly out of stock. I haven't followed the x800's as close but haven't seen them available anywhere either in abundance, if at all.
So my vote, 6800 ultra.