Im getting my 5850 this friday. Is it really necessay to change the voltage to do overclock or you can do something with the stock voltages?
You can do some overclocking with stock voltage, I made it to 325 mhash/s on one of my cards using stock voltage. You can use catalyst control center for that, or download Trixx. Trixx will also let you adjust your voltage up if you want to do that in the future.
You can easily get to 345Mh/s with stock voltages.
I run my 875/355 with Phoenix (phatk) and it is stable at 75C with fan speed around 70%. It gives me 345Mh/s.
Note that i have an 4850 right above pumping out hot air, you should get much lower temps normally.