Vista: The Upgrade
Vista improves upon earlier versions of Windows in many ways (see "Everything You Need to Know About Windows Vista" for more on its features). If your hardware is fairly new, but you're not satisfied with Windows XP, upgrading will definitely make sense--eventually.
I installed Release Candidate 2 of Windows Vista Ultimate Edition, the latest prerelease version as we went to press, on several PCs ranging from a brand-new machine powered by a dual-core Athlon 64 X2 processor, down to a three-year-old 1.4-GHz Pentium M notebook. Many of the upgrades went fine. But when they went bad, they went really bad. The final version of Vista will likely provide a smoother transition, but I'd still recommend waiting a few weeks after Vista's announced January 30th general release before you make the move. Check user forums to see what problems other people have faced, give your device manufacturers time to release new drivers, and generally allow things to settle down a bit.