It's a small bump in performance, but I'd probably buy the 6600GT. The 6800XT is the lowest in the 6800 series with only 8 pipes. (vs 12 pipes for the 6800/GS and 16 pipes for the GT/Ultra)
Only reason to buy a 6800XT is the chance of unlocking pipes, and that can be hit-or-miss. I haven't seen many threads on it either, so chances are it's not happening too much anymore.