you can also use the new CGminer, set the fans/thermal setting to 50% / 80'c using --auto-fan/--auto-clock and it will move between 850-1000 / 300 depending on the temps you want to maintain or how good your cooling is. it will also handle GPU load/crashes, etc.

i've run 2x5770 with aoclbf at 950/300 and 850/300 in a regular case, no power tweaking was needed. but, i think using cgminer makes it way easier, as you can see more of what's happening as it's happening, and it handles soft-restarts of the GPU pretty well if you're trying to find temp/voltage limits and tolerances while under load.