I've found out that the overclocker program doesn't show voltages correctly for me. I can reach 790/900 (water) without touching a voltage setting in that program. On the other hand, when I use ATI Tool I need to up the voltages manually. 1.50v seems too much for water and it doesn't scale any more from 1.475v

Be sure to kill ati2*.exe (2 of them) before starting ATI Tool and everything should be pretty straight forward.
I use a profile to initially up the vgpu and vmem to 1.35/2.1 and set default (650/775) clocks. I then have some different profiles for higher vgpu.

Also, if you have trouble with ATI Tool, be sure to get the latest 12b-version found in the beta forum at Techpowerup.

chew*:
You might need to up your vmem. With 2.1v I can reach 900MHz with BJ11 memories.