I just installed omv 2.1 on my old system an hp xw4600, e6850 3.0 ghz core 2 duo, 4gb ram, 80gb boot drive and 2x1tb wd greens(ran wdidle to extend timeout) in mirror. It uses ~90 watts (records cameras through ftp). In the future I want to make improvements with the power usage where possible. Next saving my camera recordings I have my cifs shares setup and plex up and running. Everything was pretty smooth sailing. The two big things left are having it run as a VPN server so I can remote in to my network at home and also to install Seafile to replace dropbox which right now seem the more complicated tasks. Another probably straight forward thing will be to rsync the user data to an offsite location as backup.
Kudos to the developers for allowing me this easy to refurbish my old system as a nas and not having to spend the money on a QNAP or Synology dedicated box.