Have a small epia-board always running with a couple of disks attached, this is also my music player for main sound system using MOCP. All other machines mount what they need over NFS. Disks not in use power down and the system runs from a small SSD so average consumption is under 10W.
Tried XBMC, Myth and some other stuff but it was all too complicated. (although I should say that I'm not using TV, could be that it's easier to use some mediadistro for that.)