Dual vs. Single Processors

Xanthippe asks: "I'm currently looking to build a new system and hunting for a within-reasonable-budget ideal. A friend suggested going with dual-P500 processors instead of an Athlon650. I wouldn't mind going with either, but I'd like to know how they compare. I know zilch about processors and their mysterious voodoo-god-like inner workings, but I can read a benchmark as well as the next geek-wannabe. Has anyone tried pitting a dual system against one of the 600+ AMDs? Are there any major advantages/disavatages in going with a dual system?"