MSI 880GMA-E45 freenas

My bad-RaidZ.
Letters have always confused me.

I bought that one too. It runs Linux w/out difficulty except for poor reporting of CPU temps. But if you're not overclocking, that's not so important. If it won't run FreeNAS/BSD, you could always run Linux.

Just serving files? Any processor that will fit an AM3 socket should be more than sufficient. If you want to run something like a DLNA server that will transcode on the fly, you might need a little more horsepower. If you transcode video to a format that mets the need of your client device, then it's just serving files.