Well from what I gather from the other people in this thread is, one fast processor (IE P4) is as fast as a dual processor setup (Xeon) while you are working in the app. As for rendering, I guess it still depends. The P4 with hyperthreading I've heard is quite amazing, and will probably kick the pants off any 1 ghz or so DP system (on the PC side). But I'm not all that familiar with PC's, so you might want to look into that further.

My opinion though, is get a P4... 2.8 Ghz with hyperthreading, and you'll be satisfied.