The typical answer is “It depends”. It depends on what you are using that server for. If it is an application server then you need to determine first if the application is Win2K compatible. If it is a file and print server then there are some significant benefits including DFS, better printer services and enhance reliability. If it is a domain controller then you have some significant issues to consider beyond just upgrading a server. All things being equal Win2K is a much better operatingsystem even pre-service pack. There are significant performance and reliabilty enhancments to make it a viable upgrade in almost every circumstance.
The first answer was a good answer, you should however consider your and other support peoples knowledge of 2000 against NT4, if you are using a stable NT4 system and have been for a while with multiple users conneting, things are running along smoothly what is the point in upgrading to a system that has lots of problems and a big learing curve, I have supported NT4 in large corporate WAN environments and am now part of a large 2000 project, believe me it ain’t no bed of roses and it ain’t the all singing dancing operating system that is promised (yet) there are still many bugs that cause problems.