I will have Vista February CTP available for preview at the LAN this weekend if anyone cares to take a look at it. It runs pretty well for me, I can't complain about any major slowdowns due to the GUI overhead.
Athlon 64 3200
1GB PC4000 Corsair Pro
GeForce 6800 GT OC
For our crowd I'd say that's a middle of the road system and there's still a lot of optimization work to be done yet. I ran UT2k4 on it and only had a slightly lower framerate.
If you don't care about the Aero Glass features (the stuff that makes you go Oooo), I also installed it on a Pentium M 1.73Ghz, 512MB RAM, no real graphics acceleration to speak of. It also ran just fine, albeit without all the visual niceties, but the underlying features were still there (instant search, monad scripting, Least User Privilege [which looks to be a big annoyance, can explain if you ask me], etc. etc.).
Bottom line, don't think you need a $5000 system to run Vista. Don't get me wrong. If I had $5000 to blow on a system I would, but it's just not a necessity to run Vista.