The choice of
CPU really depends on the choice of mobo and what you are planning to do. Right now, your choice is Socket 754 for Athlon64 or Socket 940 for Operton (or you could go Athlon64-FX, also in socket 940 for now, or 939 later this year).
Socket 940 offers higher memory bandwidth because it is dual channel, where socket 754 is single channel. However, socket 940 tends to be aimed more towards the server market, while socket 754 is aimed more towards the enthusiast market, meaning that you are likely to have fewer
OC options on the 940 mobos, at least based on what I've read.