You may get some improvement with an overclock of your cpu however the single most important part in a gaming computer is the video card. So while an upgrade of the cpu would also mean a new motherboard and ram if you were to get a new video card you would get a good performance increase and then overclocking the cpu would make a big difference.
One of the overclocking guides is linked at the top of this poat and there are other stickys in the forum that will give you instruction on overclocking. You can also get a lot of info from a google search on the subject. The important thing on overclocking is that it is a tedious time consuming process that requires a lot of patience and knowing what to do.