Most of the good socket 775 heatsinks are long gone.
Based on what is currently available @ Newegg i would get one of the following:
Dynatron P985 92mm Ball CPU Cooler = $24http://www.newegg.com/Product/Product.aspx?Item=N82E16835114073
Rosewill RCX-Z300 92mm Ball CPU Cooler = $11http://www.newegg.com/Product/Product.aspx?Item=N82E16835200034
I don't particularly like either one a lot since when I look for heatskink, I want something that is both very effective and also nearly dead silent. The only that fit's that those criterias is the Noctua NH-D14 120mm & 140mm SSO CPU Cooler ($90), but it is huge (may not fit in your case) and it is quite expensive for an old PC.http://www.newegg.com/Product/Product.aspx?Item=N82E16835608018
The good news is that it is compatible with Intel's current socket 1155 CPUs (Sandy Bridge and the upcoming Ivy Bridge). However, if you wait too long to upgrade, then it will be difficult to find new parts to build around a socket 1155 CPU because when Intel releases their next CPU; Haswell (socket 1150); they will basically stop producing socket 1155 CPUs.