I'd recommend at least a quad-core at a minimum..
I just put together a new server myself, however all I needed was motherboard, cpu and ram so I went with the new FX-6300, a slightly more expensive mobo topped off with 16GB ram.. With rebates it comes to just over $255..
Anyway.. it looks like the motherboard you picked will take the Athlon, Phenom X3,4,6 up to and including the new Vishera FX series.
I know the FX has been slagged pretty good, while I just slapped it together and quick threw on a test opensolaris install to make sure all is good, the machine does feel quite snappy.. I'll have to throw Windows on it I suppose to run some benchmarks, although that's likely moot because I will be running the server on OpenIndiana (Solaris).
If you don't want an FX then maybe an Athlon X4 640 or Phenom X4 955 but then you are getting near the $100 mark which puts you relatively close to the lower end FX quads.. so...