Well, it depends on what kind of a C2D you have.
An e8xxx would be sufficient, but then again you may have any other model, which in turn may not be so great.
Overclocking may bring results, but if you want to be sure to have eliminated the bottleneck, you should get a quad core, like a q9400 or better. Or you can have a cheaper quad, but then you'll have to overclock it (q6600, q8400)