SW does not utilize dual processors. I believe that Photoworks and Cosmos does use it. I run a dual process computer and am very happy with it even though I wish SW did make use of the multiple CPU's . With dual CPU's I find that whenever any application is doing something CPU intensive, you still have the freedom to run another application which utilizes the othe CPU. So rather than having to wait for something to get done, you can check email, surf the web, etc. I'd definately do it again.
As far as best performance goes, I suggest talking to your SW VAR. They have experience running SW on several platforms and can offer good advise. I'd also strongly recommend using dual monitors - probably the best bang for the buck on any workstation.