Under CPU Support it shows support for for some AM3 Cpu's. The fastest one you can get will be the Phenom II 945 BE. However, since your board only supports DDR2, you'd most likely be better off buying a new Motherboard AND CPU and Ram. You can get all 3 for 220 dollars or so, which will also bring you up to current spec as far as memory goes.
If you choose to go with the Phenom II though, I highly recommend it. I'm using the 940(945 is just the AM3 revision) Cpu and love it, but it's starting to show it's age. And I have mine OC'ed to 3.5ghz currently.