Here's what I have:
An HP MicroServer running FreeNAS for the storage. 4x2Tb drives in RAID 5 (not enough memory or CPU for ZFS) giving 5.7Tb of space.
Plex Server running on a Windows Server 2012 virtual machine on an ESX server handles the media serving duties. The VM has 2Gb RAM and one core of a dual-core Pentium G2120 host machine.
A Sky NowTV box (essentially a Roku 2 LT) is hooked up to the TV as the media player. I'm currently running Plex RARFlix on the NowTV, but there is also an "official" Roku app. You have to enable the developer back door on the Now TV to get Plex on it - it's dead easy, Google the instructions.
The NowTV doesn't have Ethernet, only 2.4Ghz wifi, no WPS, and tops out at 720p. The wifi copes fine for me though. If you need 1080p you can use a Roku Streaming Stick, but it's £50 as against £10 for the NowTV. If you really need Ethernet there is the Roku 2 XS or Roku 3.
The Plex Server will transcode anything the NowTV/Roku can't do natively. The Pentium G2120 doesn't break a sweat with the transcoding.
You set sections up on the Plex Server for movies, music, TV etc., and it goes through and identifies everything and downloads posters and the like, and keeps track of what you've watched and where you are in what you're currently watching. Once you have a Plex server running on the network, any Plex clients will automatically pick it up and present whatever the server is configured for.
In terms of power, the NAS takes around 35w, the ESX server is about the same. The NowTV's power use is negligible.
Plex Server and the clients are all free.