Mac OS X Server software is a bunch of added software to a "regular" install of Mac OS X, matched to the version of Mac OS X installed. Remote Administration (optional) requires the SAME version of Mac OS X running on a different computer.
The server software changes internal priorities all around so that Server functions are favored over Applications executed by a User sitting at a local display. It also adds additional Server software so that you can optionally run Mail, Web, Calendars and Address Book, Wikis, and many more. X-Grid controller software is included so that you can set up an X-Grid cluster for parallel background jobs if you wish.
SSH is available for remote background Terminal jobs.
If all you need is Mac File Sharing, the complexity may not be worth it, as Mac OS X itself can share files with 10 users.
I support Mac OS X Server for use in schools, where ALL User files are stored on the Server and Users sign on to any computer in any cluster and there files are immediately available. That option is of of decreasing importance in Business, as Users have more dedicated Workstations or portables that they want to carry away from the work site. But centralized backups can be helpful.
Server is generally not a compute problem, it is an I/O problem. Maximum speed on the drives is important, and having a lean-and-mean Boot drive with Users files moved off to data drives will speed things up.
In School environments where all files are on the Server, re-wiring to Gigabit Ethernet is very important to provide "near Hard Drive" speeds. Your Router does NOT need Gigabit speed, only your Ethernet Switches. Depending on WiFi alone is generally not fast enough.