In my opinion....vista is fine. I built a brand new gaming machine for $1300, with the radeon ATI sapphire X1950XT video card, the E6600 intel dual core processor (2.4GHz), and 2GB RAM. (the rest of the parts aren't 'need to know'). I decided I'd risk it, and I got vista home premium.
It works fantastic. Everything looks cleaner, everything is easy to find and use, the flip feature is great, and the media center is fantastic. Its a step down from a mac...but its much closer in accessibility. I can download stuff, play i-tunes, and play oblivion on ultra high with 0 slow downs.....its amazing.
The only problem thus far with vista is if I go to a odd-ball site, it asks me if I should cancel or allow. It also does that when I'm downloading, or deleting a program ect ect....its a minor inconvenience that I can live with and still enjoy.
I would say vista is great; but NOTE: If you upgrade to vista, there are a lot more problems then if you clean install it on a blank hard drive. My uncles a microsoft employee, so I got vista cheap, but I'm playing games like oblivion without any problems.
If your building a moderate rig, then I would suggest vista. It does seem rather demanding, but I just don't notice because my system is on the high-end of things.