The fastest was the FX-60 I believe. But they go for about as much as a new motherboard, 4GB of DDR2, and a new processor that is faster on Ebay. Your other option is an Opteron 185, but they are too pricey to justify buying as well. In my opinion, pretty much the only processor worth buying for a socket 939 system is a 3800+ or similar, or an Opteron 165 and overclocking them. My Opteron 165 ran at 2.7GHz after overclocking.