Viper Lair posted a comparison between Windows Vista, Windows XP, and Ubuntu Linux

Suffice it to say, unless you have a relatively new system in hand or you are looking to upgrade your system, don't even pick up the MS Vista box; leave it on the shelf. MS Windows XP Professional is still a very viable OS, and apparently, outperforms or maintains pace with Vista in almost every test we have run with current hardware in place. Vista has the advantage of utilizing the performance a newer setup offers, XP is falling behind in that it is not as efficient with multiple processors or large amounts of memory.

Windows Vista vs. Windows XP vs. Ubuntu Linux