OCC published a server guide, you can check it out over here.
"Hardware is one of the most important items in building a server, regardless of its overall purpose. Without enough power or memory to run the necessary applications, the server would subsequently be useless. Depending on its intended usage, however, the hardware may vary slightly. It is often a good idea to sit down and create a plan beforehand. Personally, I live by a rule of thumb that purchasing more than needed is better than having the server become congested or even fail when it is needed the most. For this guide, I will be putting together two different types of servers. The first will be a gaming server, designed to host multiple instances of LAN-based games. The second will be a file/web server, intended to back up data and make items accessible from any computer in a network. As always, hardware can vary depending on your needs; this is just a general overview of a configuration that I would use, based on the available hardware that I have on hand. Keep in mind that any computer can function as a server as long as the necessary software is installed. Therefore, you may even have old parts that can be reused for your server."