X850XT PE if you don't care much about the SM3.0/HDR features in FarCry / Splinter Cell or play Riddick, Doom3 (or plan to play games based on the engine) or flight sims (IL2, PF, LOMAC)
My opinion as a 6800GT owner at Ultra speeds. X850XT is the fastest by a slim margin without overclocking. The 6800GT/Ultra overclocks beautifully to beat most everything by the slimmest of margins. Yes, the 6800 does support some features that the X850XT do not, but that is a toss.
Buy whatever is cheaper and/or more available near you.