FIRST you need to go into the BIOS and disable any settings that will throttle down your CPU (which is why its at 6x...unless you set it to a 6x multi, which I doubt you did) EIST, C1E
(I think thats what its called). They will kill any overclock you try to implement. I can't remember all the things off the top of my head, but I am sure that someone will come in and name them specifically for you.
Once you disable these settings you should be able to overclock to your hearts content. And once you get started, the only thing really holding you back is your basic knowhow (and hardware, obviously
I got my old E6750 to 3.8 on air on my old Abit IP35 Pro. So don't set your limit to 3ghz, even with a stock cooler your looking at at least 3.2Ghz. The Core2Duo's don't produce the heat like the quads do, so your stock cooler will keep you cool enough at moderate overclocks.