Socket 754? Athlon64 3700+ (S754 version) is the highest supported processor. Personally, I used to own a Socket754 DTR 4000+ which was a bit faster, but that took FAR too much effort to run in a desktop motherboard and at the time it cost about $300. Plus, its really unlikely that you'd be able to find one.
However, you might be able to get a 3700+ to overclock by 100-200MHz.
Good Luck.