I am sure some of the experts on here will have more to say about it.I can only speak from personal experience.
Older computers or computers under 512GB of ram won't run very efficiently with Vista.If you turn off a lot of the start up features of Vista,,,it will run almost side by side with xp.
Vista seems to be more stable than XP and makes the end user have to work less to fix stuff on his or hers computer.If you download a lot of movies and stuff its easier to convert stuff,Vista does most of the work for you.
Do yourself a favor though.Unless you have a gig of ram,,,don't upgrade to Vista.STick with what you have.
