I have a non-reference (not Sapphire though) 5850 and I was actually able to adjust the voltage in Trixx. Got it stable to 1.225 and I was trying to OC to 1000/1300 but I got a hard lock. Restarted and suddenly I lost voltage control in Trixx. Without voltage control the furthest I can OC my card is 860 core and around 1190 memory (underclocking the memory does nothing for my hashrate)...I wonder why I can't adjust it in Trixx anymore, the VRM's aren't blown out otherwise the card wouldn't run...I think it might be some kind of software issue.
Ah well, right now I have an acceptable hashrate of 325 MHash/s with the card running at around 66C on load. If it's a hot day it gets up to 70C. All in all, decent, but I could have definitely broken 400 MHash/s with a 1Ghz core.