As far as I know, the settings are the same for the 6800, 6800 XT, and 6900 XT.

Lower the frequency to 1400 to 1600 at 793mv, make sure it's stable of course.

Turn on Fast Timing for the memory, then run the memory at 2100 to 2150, once again whatever is stable.

Should net you 64+ MH/s at 135W.

Also, if you have a display attached to the card, you'll have to lower the -mi (mining intensity).  The driver can crash out (assuming you're in windows), my card lost a very small amount of hash at -mi 4, but became 100% stable.