In terms of raw performance, you can pick up some older dual-socket CPUs that'll beat out modern quad cores at the same price point (though they'll use more power).
The build will probably be a ZFS storage box at first, and if I upgrade later on I might use it to host virtual machines or as a computing node or something.
Specs:
Intel x5650 @2.66 GHz (x2) - Each one consumer about the same amount of power as a 4670K and has the same raw performance (though much less performance per clock)