I have several 5850s, from MSI, XFX and sapphire. None of them have trouble clocking over 775, but if you use windows (which you shouldnt IMO), then you have to disable catalyst control center, and just set the clocks in cgminer. I dont use anything else, no tricks, afterburner or whatever. cgminer lets you control clocks, fans and voltages, no need for anything else.
Beware of flashing bioses..
cgminer didn't work for my sapphire 5850s extreme, nor did barelyclocked, msi afterburner, ccc, ati tray tools and a plethora of tools I now don't remember. All of them had some limits on clocks, between 725 and 900, only sapphire trixx worked for me (tried over 1 GHz on GPU core and worked so far, same goes for memory - lower limits available).
Flashing bioses is fine as long as you know what you are doing, it was the only way for me to raise voltage on 5850s.
P.S. trixx also support non-sapphire cards, it works on my vertex 3d 6850.
