Its am2 then?. The fastest am2 cpu is not so clear and open to debate/opinions. The fastest am2 is the quite rare am2 opteron quad core followed by the firstname.lastname@example.org windsor, but most brisbane 3.1ghz 6000+'s made on 65nm can overclock enough to beat the 6400+.There were muliplier unlocked brisbane X2's many considered these to be the fastest am2's. The windsor does not overclock so well but has 1mb cache/core and the brisbane has 512mb/core, so the brisbane needs a clock advantage to equal the windsor. Most am2 boards only support 95 watts and not 125watt. The brisbane x2 that runs at 2.6-3.1ghz is your best choice since they are 65nm and run cooler and overclock well. There is also a am2 sempron x2 that will work. I have a 6000+x2 windsor am2, i am running it on a 95 watt board and it still works fine just you cant overclock it much since the voltage maxes out on that board at 1.4v. I run it at 3.1ghz its paired with a hd-4870. It plays games well. A 4870 is about the max for that dual core cpu.
The only new am2 is a oem sempron x2 and single core brisbane sempron(be careful the sempron 140/145 is a regor am3), the x2 is a "manila" core with 256kb cache/core. they are known to overclock really well and are made on 65nm. Your only option is used if you want a windsor or brisbane x2.
It is cheaper to buy a new core2 based wolfdale celeron E3400/mb upgrade kit. They overclock well and are a die-shrunk pentium "e4400". When overclocked that would kick all of the socket 939's and am2 cpus and some of the i3's and phenom1's. The regor clocks to 3.6ghz and also kicks all of them but costs more than the wolfdale celeron. Nothing beats the $99 phenomII x4-820 2.8ghz oem in a under $140 cpu.
If you have ddr2 memory then make sure you get a board that takes ddr2 so you can reuse it to save a bit of cash.
You may find some people with am2+ boards selling am2 cpus since they upgraded to a phenomII.
Go to Microcenter and get the Phenom II x2 Bundle w/ the MSI 785GM-45 free motherboard for $99 total. In my experience they unlock to x4's that overclock to 3.7GHz half the time and if they don't unlock they OC to 4.0GHz without a problem. Then again, my experience is only 2 builds. Then you only need $50 of DDR3 (4GB) if you reuse your case, psu, dvd, hdd, etc.
I'd second that unless you already have the AM2 processor. I have an older AM2 motherboard with A64 X2 5000+BE and an AM2+ motherboard with the processor previous mentioned (under $70 @ Newegg). The difference is significant even if it remains locked.