I have an opteron and it is the god of socket 939 as far as I'm concerned. Most of them (the vast majority) overclock effortlessly, to the point that I don't even run tests after major changes in my BIOS, with the exception of my very first time overclocking. Really its all in your budget though. If you have cash I would take a look at the Athlon 4800 X2s. 2 gigs is a must if you go dual core.
EDIT: go look in the newegg promos thread in the buying and selling section.
EDIT: go look in the newegg promos thread in the buying and selling section.