As the previous posters mentioned, for gaming, dual processing offers little benefit. BUT, there are some interesting side benefits that I've learned of recently. I have a dual PIII Xeon system at my home at the moment (to do some work), and some people have given me some interesting programs that allow the RAM to be partitioned. If you have enough (this machine has 1 GB), you can partition the RAM, and run the processes independently on the processors.... so with a dual monitor setup, you can leave ICQ, web browsers, newsgroups, whatever running (and/or downloading) while you play the game on the other monitor.
But, to sum up this much-to-long diatribe.... its probably a very small advantage for gaming... one that costs entirely too much.