Well, usually people already know what they're going to do with their servers before they get one... but if you are a gamer like me you could use it as a game server for dedicated servers, Teamspeak servers, Ventrilo servers, and the like.
Otherwise it would make a good file server or a media server. It'd be best to use as a backup server to save all your files and such. But, with that kind of firepower you have under that server, it'd be really useful for an application (or game) server.
It's your choice, really.