Thermalright XP-120: 1st 120mm fan CPU heatsink :: SilentPCReview.com. Well, if you really want to be cool and quiet get this monster.
This is absolutely massive heat sink that can take a gigantic 120mm (that's about five inch wide fan). "Newegg":http://www.newegg.com/app/SearchProductResult.asp?DEPA=0 carries it for about $65. It is expensive, but worth it for the quite most likely. This comes without a fan, so you need to find a gigantic 120mm fan as well.
It works particularly well with the Globe "120mm":http://www.globefan.com/12025.htm fans. The S1202512L is their low speed 2000 rpm fan. A specialty part, they are $9 from "mnpctech.com":http://www.mnpctech.com/Globe.html. You do have to cut a little of the plastic around it to get it to fit, but that's not such a big deal.