Network Solution

We are a small non-profit organization with currently 12 computers on a network. Most of the computers store their own data, we all share the same database and connect to the internet through one of the computers. The computers run windows 95 or 98. We are looking to go to Windows 2000 server but don’t understand the licensing. Do we need a license for each computer or only the server. Currently we have MS office on each individual computer, is there an advantage to all running off the server and do we need a special license or version for that. What is the best solution?