I have a real soft spot for Windows 2000. I strongly believe it is the best operating system (or least shite) that Microsoft has ever made. It's massive set of modern, but simple to use features, rock solid stability, and proven UI make it something I wish I used more option.
If you do go with 2000, you do give up some things in the modern day. Steam might be one, but I haven't checked recently. You also give up support for some earlier Windows 9x games that weren't written so well. NTVDM is also so laughably bad that it caused an entire forum to spring up around getting older games to work on it. (I wonder which one that is... hint hint)
Either way, if I could, I would use Windows 2000 today. It's truly a wonder to behold, and I wish we had stuff like that today. XP might be the more reasonable option, but it could also end up being the slower option. XP had a decent amount of extra overhead compared to the relatively simple Windows 2000, so that might be something to consider, even on a dual CPU 1ghz system.