Someone can correct me if I'm wrong but I don't think you can use that CPU in a dual proc configuration. You'd have to go with something like the 2609 for that same price but that's only a 2.4ghz part.
Oh man, that does seem much more sensical. I did not even think about Xeons not having the ability to be paired.
I would say i7-3930k, but what programs do you use? 3d?
Cinema 4D and After Effects are my bread and butter. I am in the process of learning Maya, Nuke and Premier.