I would not upgrade an existing computer to Vista , rather purchase one already running Vista.
You do need a chunky computer with at least a GIG of RAM, and a pretty fast chip, and a top end graphics card. But even if you do have all that you may have driver issues.
On the face of it , it does look like an improvement over XP, better search facility, nice graphics, supposed to be more secure!
Downside is the DRM ( Digital Rights Management ) that Vista comes preloaded with.
It's down to the ability ( assuming a HD player is present) to playing HD content. It's locked down real tight, so tight it's causing problems playing "normal" DVD's, reducing the quality.
Even it you don't have a HD player installed.
Part of the reason you need a fast computer is the constant checking back to M$ to check your computer is all legit and not doing any funny stuff to try circumvent the DRM.
I would imagine that it would be best for a quiet life to wait till the inevitable " Vista SP1" to arrive and sort out the glitchy bits. Including an easing of the DRM as people get pissed at the inability to play your DVD's properly.
Looks nice but !