Avoid XP Home if possible - many important features are not easily accessible.
I use XP Pro and 2000 Pro for my work and have found that the main advantage of XP is the ability to switch between user accounts "on-the-fly" - i.e. without logging users off. Very useful if several people will use the machine - you can switch between accounts without each user having to shut down their applications.
Aside from this XP doesn't seem to do much else that I need to do better than 2000.
As seacue has noted searching for files in XP is a pain. Also I find it much easier to configure user accounts and file shares in 2000.
As for minimum system requirements - Microsoft say 133Mhz and 64mb ram for W2K Pro, 300MHz and 128mb for XP Pro.
However, in practice the system will be unbearably slow with less than 128mb for 2000 or 256mb for XP.