Keep in mind that overclocking can only do so much to boost performance due to the Von Neumann Bottleneck.
http://en.wikipedia.org/wiki/Von_Neu...ann_bottleneck
Overclocking is also kind of risky because a processor is given its speed rating for a reason. You have to keep in mind that even with a good heatsink and cooling system, it is still really easy to fuck up the CPU due to the extremely hot surface temps that can occur. If you do it, do only small increments of frequency increases at a time.
I doubt you are doing any intensive parallel computation other than gaming so I recommend that the best place to invest is in RAM and a top of the line GPU.