I have a Gigabyte Radeon 5850 and, using AMD GPU Tool, run it at 875/1150 without heat or stability issues. If I run the memory even 1 Mhz higher, I run into lots of issues but that is pretty standard for ATI cards. I have always found the memory to be the biggest limitor in respects to Overclocking.
There is an article in the Round Up on the splash page from last week (I think) where Overclockers.com pushed their 5850 to pretty high numbers by increasing the voltage a bit. Myself, I don't feel the need to push it that far so I only run the higher clocks when I plan on running really intensive games or when I am benchmarking. Otherwise, I just leave my frequencies at stock.
Good luck!
Happy_OC_Canuck :)
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