Install FreeBSD on it (it's very similar to OS X), configure as file server to back up your Macs, make it your proxy server, firewall, etc to increase your secu

Install FreeBSD on it (it's very similar to OS X), configure as file server to back up your Macs, make it your proxy server, firewall, etc to increase your security. Since you will probably run it headless most of the time, if it has integrated GPU, then you could sell the GeForce card.