Do you want it to be :Windows Server" type of server, not Windows XP shared like server. Or is that what you meant?
Because, for instance... if you want to play games install anything but SBS 2000/SBS2003 on the box. It's not really designed for playing games. More to handle applications, etc. Make sure you don't go that route!
what games? There are many Linux dedicated servers for many different games. You can also file share and do other things on it as well. The pluses are free open source OS, doesn't require lots of hardware so you can keep your costs down. The downside is you would have to learn how to use linux (in all honesty it is not that hard)