+1 on Skuzzy's remarks: a fresh install of almost anything (well, maybe not Vista) is likely to result in a cleaner system with less background stuff than a 2 year old installation of anything.
Now with all that said, my thinking is this:
If your new system comes with Windows 7, don't bother looking into reformatting the drive and installing XP. Win7 is good enough and the driver situation has now settled down for the most part. The only 2 exceptions I can think of is (1) if you MUST use old hardware that isn't supported by W7 or Vista and you can't easily or cheaply replace it with something newer -- Gameport joysticks come to mind, or (2) you need to manipulate audio or video in a manner that Win7/Vista's DRM-enabled pathways prevent you from doing.
If you have a system that's newer than 1 or 2 years old with XP and you want Windows 7, just buy a second hard drive and install Win7 from scratch onto it. You can still dual-boot to run any old software and games, but then run Win7 for all the new stuff.
If you have a system that's newer than 1 or 2 years old with Vista, just do an upgrade - it works pretty well. If you're at all concerned about the upgrade messing things up, then get a second hard drive for W7 as above.
If your machine is older than 2 years old, just stick with what you have, unless your 3-year old machine was a top-of-the-line $4000 monster.
-Llama
