Windows XP and 2000 are essencially the same OS, the difference is more down to the technologies being employed.
2000 is far more stable, especially at the kernel level because it's got less to go wrong. Unfortunately there are some limitations with it that mean it no longer is viable for the modern technology.
(good example is lack of multi-core processor support, not to be confused with multi-processor support)
I think honestly 2000 is a better OS, particularly as it can be cut-down to the absolute bare essencials without sacificing program compatility. That said none of this matters as Microsoft are going to force Vista as the new platform regardless..
Intel Core 2 Duo E6400, 512MB DDR2 667MHz, ATi Radeon X1900 XT 256MB PCI-E, Windows Vista Business / XP Professional SP2