It won't do much for you right now.
Software has to be written to take advantage of many cores, but this is a far from simple thing. Software development has for the main part, up to a few years ago, been single core.
Mac OS X (Tiger and Leopard but especially the upcoming Snow Leopard) has some support for dual core systems. Windows has a little less support (especially with respect to Snow Leopard). Linux can utilise several cores, but again the software has to be written to make use of these cores.
The idea is that any parallel tasks can be put on several cores. You could even keep one core free for interface processing, and the rest for actual computation. That way, the display always feels responsive to you.