What is needed to create & maintain game server?

I was thinking of setting up some game servers and was wondering if anyone could explain what is required to accomplish it. Any input would help.

Sounds doable. For some reason I though there would be more to it.  In a few weeks I will be coming into some decent used higher end PCs on the cheap & need a project. When I get 'em up I will announce opening to everyone. Thanks for the info. 

I was thinking about asking community members for input to see if there was a couple we all could play.