CGminer runs at about 512MH/s on my 5850, the clocks are 925MHz and 500MHz, but the GPU stops responding after about 20mins and CGminer declares it sick, then dead. The cooling is handled by an aftermarket GPU cooler, not the stock XFX one.
Bitminter yields a more reliable hashrate staying consistently at 350MH/s at the exact same clocks going to about 400MH/s for about 10 seconds then comes back down to 350MH/s about every 5 minutes.