Windows 98, ME, or 2000 on a new system

I just finished building a new system for work related applications. No intensive graphics (mostly MS products). running on a peer to peer network.
800 thunderbird AMD processor, 164 MB ram, 45GB HD. What operating system is better. Why? Can I use Win 2000 server as a stand alone, why or why not? If I set it as my server, can I still peer to peer, and use it daily as a main computer with each type or operating system?