Windows 2000 is cheaper, much more secure, slightly more stable, and doesn't deactivate itself after making hardware changes (or in 2 years when Windows XP users will be forced to "upgrade").
Windows XP is still full of bugs. Windows 2000 has had 2 service packs and a security pack to date. (You will need to install service pack 2, which contains all of service pack 1 fixes, after installing Win2k.)
About the only thing going for Windows XP is that it looks newer IMO and comes bundled with a whole bunch of security compromising utilities enabled by default. (Remote desktop and universal plug and play to name a couple.)
Regardless of which OS you choose, look at
http://www.ntcompatible.com . They have lists of compatible and incompatible software and hardware with the NT based OSes.
Windows XP will never be on my computer (nor will any future Microsoft OS) as long as Microsoft insists on pursuing their "product activation" feature. When I buy a piece of software I expect it to continue to function until I decide I want to upgrade or replace it, not when Microsoft feels like I've made too many hardware changes or after 24 months.