i have all 3, XP Vista, and windows 7 RC on 4 machines. my bottom line is if you are running XP and are not having any problems stick with it. its familiar, its tested,its compatible with everything, No driver issues and works with everything.
Vista is more like Windows ME..... it was pretty and fluffy and tried to simplify everything but in the end it really didnt add anything from a users standpoint. it was a resource hog, had a decent learning curve to it, and had alot of compatibility problems. However it was stable but like windows ME it didnt offer much for the price. i tried to sell vista ultimate on Ebay for $40 and had no takers. seems no one wants it even me.
Windows 7 is very good and very stable. it also offers virtualization and allows you to run XP application in seamless windows XP virtual envirerment.(this means you can run xp programs in XP inside of win7) it also offers security enhancements and is very stable. it has driver support built in for most hardware and many other enhancements. Windows 7 is a great system for users but if you are someone who doesnt do much but internet,emails, office and some games its hard to justify spending the money for it. i am running win 7 RC with 8 gig but even gaming i never use more then 3 gig of ram as my graphics card takes a big hunk of any gaming load.
Xp still does what Win 7 and vista does and its cheap and already on most systems. My opion is if you get 7 with a new computer fine but not worth spending money to buy or upgrade for.