So finally got my Sapphire 5850 yesterday, and have been lucky enough to get it pushing 403.35MH/s at 1020 core / 300 memory clocks, using Phoenix / phatk on Windows XP, SDK 2.1 and 11.5 Catalyst. Had to use 1.2 volts to get there, but my core temp is ~70c and my VRM temp just under 80c.
Using GPUTool (old school Furmark, essentially) to test for stability, this is no where near stable for actual 3D graphics / gaming. However I only get rejected blocks when they are submitted around a block switch-over, and only had a driver crash when pushed up to 1030 core - otherwise I'm hashing away happily.
Is stability above the maximum gaming overclock because only part of the graphics core is used while mining? And would the shares generated by Phoenix show as "rejected" if they were invalid due to an unstable overclock?