Tisk tisk.
First of all, it entierly depends on your machine.
That said, a few corrections.
Yes, XP does boot faster, and yes, it does use a little more system, but it's worth it.
With some tweaking you can get it down to 2000 Pro efficiency.
It also supports newer hardware, handles RAM better, and more efficient with running programs.
Many think that 2000 and Xp are the same basically, which is mostly true, only XP has further optimized and upgraded etc those components.
Anything under 1.3GHz-1.5GHz and 256MB RAM or lower I'd go with 2k Pro, anything better than that I would go with XP Pro.
Yes, when you install XP you must activate it and you can only have one license key actived on one computer at a time. However, certain licenses don't require activation, such as the Corperate Edition licenses. If you're going to college or something, you might be able to get that from them for a few bucks. My brother can get a single license version of XP Pro for $5, but that's the kind that requires activation. Still a good deal.
From my experiences, XP has been just as stable as 2k, if not more so.
And, it's pertay...
One thing I don't like about XP...It's a lot harder to do things manually, and I don't like that, but I haven't had a large problem with getting it to do what I want it to do, just sometimes requires more work.