Vista does ICS better than XP. A lot better.
... that's about it. Whatever new features that were actually good were the ones to improve performance - except Vista itself nullifys any true positive effects of those. XP (Oh, yes, I love TinyXP) runs a hell of a lot faster, and you aren't losing ANYTHING; Besides maybe DirectX10, but... eh, not worth the trade. The best idea would be to, erm, "obtain" XP beforehand so thusly when you get your new PC, you can kill that pathetic mammoth of a OS.
... People here are mentioning Linux. If you want to look at something [not quite as] different [as you would think], I would suggest trying out a few distros like Ubuntu (ugh) and Puppy on a LiveCD and seeing if you like those or not. Linux is becoming more and more userfriendly every day - if you can handle the hassles of Windows XP, you can handle a desktop Linux. They're not the same thing, though, so don't expect everything to work the same way it does in XP; XP's methods aren't the only logical ones, you know.
Essentially everything minor (not gaming) in XP you need to do, you can do in a modern Linux, so I'd still give it a shot on a LiveCD if only for the different experience. SRB2 wise, I recall most things working fine on WINE, but Linux is not a very SRB2 friendly OS.
Oh, and please, if you're getting a new PC, make sure it has a PCIe slot. Or a PCIe video card of some sort. Never, ever get integrated graphics.