I've just bought a new shiny quad core with vista installed, I was a bit duboius but so far its run HD editing with vegas 8 pro very, very well. It's the sheer speed of rendering compared to my old computer which must be mainly down to the increased processing power.

At the opposite end of the scale I've done a fair bit of HD editing on my Celeron single core processor (stop laughing) and although slow it can be done, just don't tryt anything to fancy!