Here's a home theater PC (HTPC) "bare bone" case from LixSystems, a company that apparently pays attention to the right details for a box that's going to be in an environment that needs near-total quiet. Its 280-watt power supply has a fan that keeps the noise down to 14dba, which is right down there in church mouse territory.
You supply the processor, hard drives, tuner card and RAM, and for $299 LixSystems provides a case that's just 16.2"x11"x3.2", about the size of a DVD player. It's equipped with a motherboard for AMD dual core processors, onboard 5.1 audio and NVIDIA FeForce graphics support, a 9-in-1 memory card reader, and interchangeable front plates to match that stylish home theater of yours. Choose Windows or Linux, and you have yourself a quiet and low-priced foundation for a kick-ass home theater PC.
Product Page [LixSystems]