I have a HP N40L little 4 disk SMB server which runs ubuntu perfectly with all the tools you list along with RAID and a bit more power.

Instead of having XBMC I use plex a free (though the plexpass is so worth it) fork, which keeps a central library which clients then query, they have a client for everything (rpi, android, ios, windows, mac, linux), by far the best feature is that it remembers what point you were in movies and has a great web interface where you can watch anything - no installation of anything required. Also syncs content of your choosing to ipad or tablets/phones so when you hit the road you have all your shows you want at the correct quality and encoding - I freaking love it. If you have a good enough speed uplink you can also even watch remotely in your browser.

If you like XBMC - try Plex, it's worth a little look at...

edit: Also runs on Roku's too :)