I’ve tried this and it works well, but it’s expensive. Johnny Winter:
The Mac mini is an excellent candidate – for a media server – either connected direct to your television or used remotely. It runs silent, draws little power and can be upgraded relatively easily if you are confident and competent with paint stripper (yes, seriously!) and a screwdriver. Even if you don’t fancy getting handy with tools, plugging in a large external drive to house your media is sufficient, even over USB 2.0.
Nice comparison of a Mac mini vs. an Apple TV.