Great review site: http://www.smallnetbuilder.com

I could use http://freenas.org with an old dell server I have.

Must haves:

1. RAID 1
2. The size of the drives should not be limited
3. Media Server

Do I care about having AD?
Nice to have power management so its not always running 24/7.
I don’t think FTP is important
NFS/CIFS/SMB – Its important to be able to mount it from any OS