Products, offered by UK based company Aleutia, can be good candidates to be HTPC or media client. They are tiny, noiseless and use very little power. However they have enough power for HD video playback thanks to nVidia Ion graphical platform.
Also you may peep inside the Aleutia’s PC using their Flickr photo stream. And even you won’t buy anything from Aleutia it might give you some useful ideas to build tiny and silent HTPC by yourself.