What desktop o/s to use?

A small firm of 20 (growth to 30) has admin, clerical, engineering (who require access to a mapping application) and finance all require access to a database. The intention is to migrate to Win2K advanced server. Some desktops currently run win95. What are the main reasons for upgrading desktops to win2k professional rather than 9x, NT or XP?

This is one small part of an assignment I am currently studying.

TIA.