Vista has run rock solid from day 1 and is as responsive as XP ever was for me. I mostly play games and do web dev stuffs and I've never run out of resources in any situation. Well, okay...there was this one time I was being indecisive and had 20 porn videos open at once...that slowed things down a bit.
To anyone who's going to be installing Vista or getting a new Vista machine soon: use
Vlite. I shaved almost 2 gigs off the Vista install DVD with this.