^ When something works, no reason to change it. I've still got two rigs running on XP, one on Vista, and three on Win7. Vista is now almost as good as Win7 (talking about standard performance, not things like Trim for SSDs, etc.). XP is . . . well, XP. It just works.
No point. There are a few nifty things in Win 7, but I wouldn't consider it worthwhile to back up, format, and reinstall everything. If you must have XP compatibility mode, then yeah you may want Win 7. I have one desktop machine with Vista business though and I just run XP in Virtual Box. I do the same for my Laptop with Win 7 Home Premium. Keep in mind though that XP mode doesn't have any GPU 3d acceleration while you can run old XP 3d games in a Virtual Box VM of XP.
Personnaly i moved straight from xp to win 7 64bit i jump vista because of all the bad hype before it came out and i am glad i did. i use my computer for gaming and have to say that win7 is the best os i have used and can not fault it in anyway i would highly recommend the jump to win 7 and i dont think you'll look back after you get used to the different interface.
It just feels smoother looks great and works. as to whether its faster than the others i'd say it probably is but have no stats to back it up i am sure there probably is if you google it.