The differences are vast.
Windows 2000 Professional and Windows 2000 Server line benefit from the code base of NT with great enhancements/rewrites to code massively increasing the reliability, performance and security. From the GUI prespective they many look similar, but don't be mistaken. They are not several hundred dollars more purely for differences in GUI, boot time and included user applications.
Windows 2000 Professional is intended for workstations and 2000 Server is intended for production servers, be them departmental or internet site serving or whatever.
IIS on Pro is limited to iirc a certain number of sites (1?) and certain number of concurrent connections.
File serving - Professional can serve upto 10 (or was it 25?) concurrent sessions for file sharing, Server is restricted by hardware capabiliity (and licenses).
AD - Server can function as a domain controller or domain member. Pro can only be a domain member.
Server can also function as DNS server, DHCP Server, WINS Server, DFS Root, Full Print Server, Gateway to Novell and Legacy systems, RRAS, Certificate Server, etc...
You really can't compare them as they both serve different audiences. Windows 2003 is again a server line, so it will be of little benefit to you, if you're on the beta program then have a look at Longhorn, I've not used it myself but have seen it in action, looks interesting.