Make Magazine has a cool article about taking a PC and building your own media center - right down to including a gaming system using N64 controllers and emulators.
One of the cool things I found was the streaming music using SlimServer, although I tried to set it up, and while I could see the streaming going, there was no music. However, I think it would be cool to get one of their devices, and use it to stream music through my receiver, since I don’t have any immediate plans to build a DVR.