I'm running Vista Ultimate on my year old compaq presario 2610 Laptop, and it's not as much of a resource pig as some people were screaming Vista would be. My Compaq needs an upgraded HD (Only 40 gig) now. Also I have just over 1 gig of RAM Vista is actually running better that XP Pro did. How's That? :-)
I've been running Vista on this machine since beta 2, and had a few bugs, but no performance issues.