I'm not sure its really worth spending $30 for a heatsink, that processor itself probably isn't worth $30 from a performance standpoint. A can of compressed air or a vacuum + attachment to clean the dust out of the heatsink will likely bring it back to its former cooling glory. My experience with OEM cpu's was that overclocking was usually more hassle than it was worth, and the motherboards are only slightly more advanced that corrugated cardboard. Your computer has the intel 945g motherboard, and as this lengthy review shows, there are no overclocking options in the bios
http://www.pcstats.com/article...?articleid=2011&page=5. You could try a software overclock in windows but I think you'll be wasting your time. You may be able to overclock it a touch, or you might not.
To conclude, vacuum out the dust, if you need more speed save the money for a new processor. That motherboard will support a core 2 duo if that's what you want. A core 2 duo with a voltage/fsb mod would be pretty badass.