OP, and to anyone whose Mhash isn't what it is supposed to be and goes down with overclock.  In your task bar right click on the ATi CCC icon and go to performance I think it is. There is a power saving feature (powertune) set it to +20, then apply settings.  You should now see a huge increase in speed, my unlocked 6950 was doing 360mhash/s before and now that I set powertune to +20 I am getting 400mhash/s.  Good luck!