Dual processing was very cool back in the days of the P3 and P2s. Today though, it's not what it was. The only Dual proc that do anything reliably well are the Xeons and they are way expensive. AMD has thier dual systems, but every Dual AMD put together that i have seen has had troubles. But that has been a while now. The new Opterons are supposed to be pretty amazing, but I thought I saw something about them all going to single and the 64's moving on with dual and quad possible down the road. But I could be mistaken.
anyway, the Dual processing systems that I have used, P2 and P3's have all done very well. Dual processor boards tend to be higher end then the single and have more server like options. Comppaies like SuperMicro and Tyan are the best when it comes to Dual, as they pay close attention to every detail and provide awsome support software for the boards.
As to actuall performance, Running 2k and XP you will see a large imporvement on most things. Business apps all use Dual instructions now, as the original dual, Pentium Pro, paved the way for it. A lot of games have Dual instructions too, but they just don't talk about it much anymore as it has kind of dissapeared behind the GHZ bandwagon. Although, with the release of the P4 with Multi-threading the Dual systems are looking less and less needed.
I still run a Dual 1.26 p3 system (2.52 combined) that kicks the crap out of the 3.2HT system I play my games on. Now this is with running Autocad 3d renders and doing FlashMX and 3ds max stuff. Also, I can re-encode video about 4x faster on the dual system as compaired to the 3.2HT. So it kind of depends on what you are wanting to do with the system I guess.
Hope the helps you. I know I rambled a little...