Linux.
Then you can have it as a media server.etc if you want to.
Apache + Php + jinzora makes for a great Jukebox, bit slow sometimes (that might be down to my server being a p3 500) but works with pretty much any media player that supports HTTP streaming.
use webmin to setup shares.etc makes it lot easier than playing around in the console (unless you wish to play around in the console of course)
Freenas has a nice web interface but its a bit limited IMO, if you got the machine running make it more than just as NAS
Those sata controllers you can get for about £10 on ebay seem to work ok (or at least the one i got did) if you find yourself short on ports to plug the HDD's into.