Upgrade of server

I have been charged with finding out how to upgrade our PC Server. It is a P3 workstation running Windows 2000 Server. With about 15 users. The users store their infomation from Excel, Lotus 123 and Word.

The powers to be want to upgrade to a true server that will take them into the future 2-3 years and allow for growth.

Any ideas that can get me started would be great. Thanks