How far a system can be overclocked depends on so many factors. A proper cooling system of course helps to get higher and more stable overclocks, but the main factor will be the CPU. Every CPU is different, and the same CPU model can behave very different from one unit to other.
This said, your CPU model should be able to do from 3 to 3,6Ghz without so much effort (again, it depends on the CPU itself, as well as the Motherboard, memory, PSU...)
There are plenty of Overclocking guides around for C2D CPUs, just do a quick search and go step by step. It is better to start with a modest OC and keep raising it slowly, than to blow up a CPU because you tried the maximum values that your moderboard lets you to set up.