Though I know you might be aware, Plex is actually designed for this. It is derived from XBMC source and runs in a separate client/server model. I used XBMC for years but when I added more clients (now up to 3 Roku, 4 iPhone, 2 iPad, a MacBook Pro Retina and a few Chromebooks through PlexWeb), the Plex setup was much simpler to administer.
Plex Media Server runs as a plugin on unRAID and works exceptionally well. I'm still a huge fan of XBMC, but it does not meet my needs as well. If you are running a single client or want separate HTPCs at each TV with separate libraries, XBMC may still be a better fit.
Plex does have some disadvantages: no smart playlists, lack of PseudoTV functionality. But the advantages, for me, were too many to pass up:
automated sync to iPhones/iPads
centralized library management
transcoding of files on the fly for different client types
Excellent Roku support
And I am in no way associated with Plex. I just hate to see someone get frustrated trying to get XBMC to do something which is more easily accomplished with something else.