technicaly there are the server, and the advanced server.
they differ in the amount of memory that can be used (4GB for the server and 8GB for the advanced server), and the number of processors that they can handle (4 for the server, and 8 for the advanced).
Above these two ther's still a more powerfull w2000 os, called the datacentre server, which can hold 32 processors and 64GB of Ram)
Commercialy you have the upgrade (nt4 to w2000 server or advanced server)
the compettitive upgrade (novell to w2000)
and the full version (nothing to w2000)