Now this I can answer: Athlon MP processors are tested to run in SMP configurations (multi-processing) and, if I remember right, usually come unlocked (someone back me up here?). They cost, on average, about 50 bucks more than the XP version. The normal XP version is what you would want if you want a normal single-processor dektop CPU as they are cheaper.
You only die once, don't mess it up.