Vista will take advantage of 64-bit processing whcih means things are done alot quicker.
Go Dual Core if you want to "future proof" yourself. Go SLI/Crossfire. Its a way to set yourself up for years to come. Heck, I'm still running on an Athlon XP 2500 at 3100 speeds (mobo hates 3200 speed for some odd reason) and a 9800Pro from 2003/early 2004 and it still kicks good enough in new games. Can't turn everything on but it still plays them very well, considering I do have 2 gigs of ram
But yeah, as said, go Dual Core. Though right now its not used as much, Intel and AMD WILL go dual, triple, etc..et.c..cores in the future. and companies will show the advantage of dual vs single when the software is used right.