If it's just going to be a NAS then I'd run FreeNAS.
Plex/DNLA server plus a few other things too eventually.
You can run those in jails/VMs (these aren't the same in FreeNAS11), they even have a few standard issue things available as 'plugins' that come ready set up (e.g. Plex).
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If it's just going to be a NAS then I'd run FreeNAS.