Give it time. If you've got Windows, it will slow down.
I've built it over a year ago and I've yet to experience any slowdown with Vista.... or any sort of problems in general. I must be one of the lucky ones. Had a spot of trouble with BSODs at the start though, but turned out my RAM was faulty since Ubuntu and OSX kept crying about it too.
Might upgrade it again, get a decent motherboard that has SLI and dual CPU sockets or something like that. Might even get a dedicated PSU for the graphics cards, if that's possible.