Actually, windows XP Pro and 2000 Pro are almost equal in terms of being up-to-date. This may change when the XP SP2 is released, possibly later this year.
The major difference is with the crippled XP Home version which lacks the ability to log on to a network with a domain name, lacks file encryption, lacks multi-language support and lacks multi-processor support.
One notable difference is that windows 2000 has slightly better support for some legacy devices than XP Pro. As an example, my KT7A-raid system uses a highpoint 370 raid controller that is supported in Windows 2000, but is not supported in XP without an enthusiast modified controller/motherboard bios. Windows 2000 was designed to be an upgrade for machines that had been running NT since about '96, where XP was intended mostly for new machines built after its release (with some upgrades).