If this is a dedicated mining rig then run Linux and don't unlock the second core. You don't need it an it just generates more electrical load. I actually downlclocked my semprons to 2.0GHZ cutting 10W off the load. Yeah 10W isn't much but every watt helps. I also turn off onboard audio, USB3, SATA controller, serial port, etc as well as downclock the RAM. It all adds up. I run 3x5970 and entire system draws 860W at the wall.
Now if you are running Windows (100% CPU bug) or this rig is used for more than mining then you might need the second core.
but how can I overclock on Linux?