Intel Xeon E3-1231 v3 or Intel Core i5-4690

[NKF98]

The i5 has a significantly better track record than the Xeon, especially that late of a generation (4690k is an excellent choice for this application) although you might want to reconsider that MB... you won't realize the full potential of the processor, better to get a more expensive board and have the ability to OC. Also, unless you have a dedicated GPU, that mobo won't do you justice.

EDIT: The wiki says that the i3 runs SSBB at playable speeds, so 4 player should be no problem for what you've got.