Hold on there. When you say power, I trust that you mean overall performance and not the voltage of the GPU
Use whatever is stated below AT YOUR OWN RISK.
I'm no expert on overclocking, but get ATiTool (beta 12 or 13) from guru3d.com. Install and run. You'll see the following screen.
Use the sliders for core and frequency and increase at 1-5mhz at a time and run the artifact scanner - BEWARE, USE SMALL INCREMENTS AND RUN THE ARTIFACT SCANNER FOR SOME TIME - this will allow your gcard to heat up and reach operating temps. Please note that the artifact scanner does not compute as fully as some recent games, so results may vary in games in terms of temps and stability. Once you reach a point where artifacts occur, drop down 5mhz or more on core and memory speeds and test again.
Or just go for the automated way with Find Max Core and Find Max Mem (reccomended) This will take some time and don't skip this part, let it finish by itself.