While dual processors will let you reap the benefits of SMP, your software has to be written to take advantage of it. If you're building a gaming machine, don't bother. Most games are unable to take advantage of SMP.
Several years ago, when the first Celery 300A's came out, I built a dual processor machine overclocked to 550 (X 2). It was fast for its time but if you looked at processor usage during gaming, only one processor was being used at one time. What was funny was that sometimes one processor showed 100% utilisation and then the other one showed zero. It would switch (seemingly at random) and the the second processor would show 100% utility and the first one would show zero. Either way, only one processor was being used at any one time.
So unless you plan to build a server don't waste your money, get a hyperthreading processor or one of AMD's latest offerings.
-Target