lews 3200 on a 754 socket or 939 socket. On a 939 socket you can uograde to the fx series after a year. Also my sources say the 939 socket will be around for a long time(1,5-2 years) so any CPU that comes out for the 939 you can put in the MB as an upgrade. Start with 3200 end with 4500 or more.
ps: seen my MB link a few posts back. also remember that to fully exploit 64 bit use Windows XP 64 bit edition.
or you can wait till the dualcore cpu come out.