I used to run 2 Sapphire 5830s at 950/300 for stability under Windows. I found a tool called aoclbf and I used it for over/underclocking and it also has a way to configure miners. I had more luck with GUIMiner and phoenix and got around 310 mh/s from each card. Temps stayed in the 70s and each card needed around 140 W from the wall if I remember correctly.