with that CPU, make sure that
A. you have good enough cooling
B. powerful enough PSU
then its simply a matter of upping the multiplier in the bios, chances are youll make it to 3.8 stable without any voltage changes, but there is also a chance that you might need to up the voltage a bit.
check tom's overclocking guides for further details on how to do this
Unless you're running a pair of GTX 480's in SLI, it should be more than enough. The PSU doesn't just power the CPU - it powers everything. So, you have to take all the system components under consideration.
At it's stock speed, that CPU only draws ~80W of power. If it manages to unlock the two disabled cores successfully, at the same stock clock speed, it should draw around 125W. Overclocking will increase it's power draw beyond those points, but how far beyond them depends entirely upon how aggressive the overclock is.