I found a nice comprehensive review of the QNAP SilentNAS HS-251 over at nikktech.com As its title implies, the QNAP SilentNAS HS-251 is a two drive passively cooled NAS. It has no noise making fans, and is in fact totally silent except for the noise of the hard drives you choose to use. It is built into a slim case that would look good in a media rack.
Specifications include that it is Intel Celeron 2.41 dual core based and its software includes the popular XBMC media player (now renamed to "Kodi" by the way).
The review contains a full hardware breakdown with photos and a whole page of test results. Needless to say, the NAS is plenty fast and should work well as both a media server and a media player.
The NAS is both silent (except for the hard drive noise) and has very low power consumption. The only drawback to all that silent performance is the somewhat high price for a two drive NAS. But since it performs as both a file server and a media player, maybe the true value of this device should be considered very high.