I work as a server admin, and I have an office network with this very set up. I use a Dell computer with Windows Server 2003. The other computers can view its shared drives, as well as logon using Windows Remote Desktop. This enables a user on another computer to work on the server as if they were looking at the server through its monitor.
If it's
only for shared files and printers, I don't see why you couldn't use XP or Vista right now on your server tower; but, if you ever wanted your local network to get any more complex or advanced, you should get Server 2003 set up. There are some excellent books on learning how to set up Server 2003 very fast and efficiently.
This one especially.