I've got a custom-built HTPC in an Antec Fusion 430 box.
Cost me about $400 to build (+$65 for Harmony remote). It's nearly completely silent. I hook up my Shaw HD box through firewire and use it as a HD-PVR. The only software I use on it is SageTV; it's the ultimate HTPC software as far as I'm concerned. Videos, TV, pictures, music, online video, etc. Cuts out all the commercials in recorded shows, compresses anything older than a week into h264 files, and simple enough that my girlfriend has no problems using it.