Yes I know it will run all the way up to El Capitan. I'm using this machine as a server (plex, file sharing, VPN for when I'm away on unsecured wifi, torrentbox). I was going to keep it old school and run 10.6 but I was wondering what you all thought the sweet spot for performance would be for this machine. Perhaps Mountain Lion, or Mavericks? Running 10.6 it's just a hair too slow for plex server to do high bitrate 1080p transcodes. I was hoping (perhaps erroneously) that something like Mavericks might have better performance. I've included a poll as an experiment but please post your thoughts. Thanks.