I don't really think so. The cost of the more expensive motherboard and CPU just aren't warranted by the performance gains. Unless you do an insane amount of multitasking, it won't help very much. In the vast majority of games, it won't help at all. They just don't have multi-processor support. Certain applications like video editing programs, image editing programs, and video encoding, the extra CPU will definitely help. For the most part, today's single CPU systems are even overkill for the majority of applications. Don't do it unless you do professional level work on your computer.
lke everyone said, it deoends entirely on what apps you use, and how hard you use them (i.e, do you just like messing around, or do you spend hours a day with them?). for some apps the diference will be night and day (i.e., rendering large scenes or movies in 30-40% less time), and for [most] others, there will hardly be any diference at all.
I like having the computer be really snappy all the time regardless of what I am doing, that is why I like dual-cpu systems. Had the BP6 for a while and now have the Tyan Tiger with dual 1900+ and a healthy 1.5GB of PC2100. Nice and snappy indeed.
The way I see it, you sit down to use a computer that costs maybe $2,000 if you add in all the components and accessories (printers, monitor, digital cameras, software, etc... you know all that stuff), so you spend an extra $200 bucks (extra $100 for MOBO and extra $100 for a 2nd processor). Now you spent $2200 (whoopee, an extra 10%, just don't go out to dinner or order pizza five or six times and it was essentially free) and your computer is really snappy regardless of whether you use multithreaded apps or not. If you already spend so much money anyways, I figure it's worth it to spend and extra %10 and gets a Rolls rather than a K-car.
Honestly, even if you do use Photoshop and other apps that take advantage of dual CPUs I don't think it's worth it. That is, unless it's a professional job for you or something. But if it's a hobby of yours that extra performance of those few apps doesn't warrant the extreme cost difference. Now if you have tons-o-cash then it doesn't matter, eh?