What kind of home theater PC can or should I get for less than $400?

I would like to get a PC to use with my 42" LCD HDTV. I want to use it to stream HD internet video, playback stored media, browse the internet, and for word processing. I'd prefer a small and quiet PC that can be mounted behind the TV and to keep it under $400. Can this be done? Anyone have any suggestions? Any reliable PCs that meet these requirements? I don't necessarily need a DVD or Bluray drive..