I've heard nothing but praise for the Opteron 165.
It's clocked at 1.8 ghz, but has the 1mb cache per core that the 4400/4800+ has. It's been easily overclocked to 2.4 ghz from what I've seen, and it's cheaper than all of them.
May as well go with that...
EDIT: The 1xx Opterons are all for S939, so don't worry about that.