I keep at least 2 copies of all of my important files on separate machines, and if it's "mission critical," it gets backed up to a portable drive and burned to a CD.
If I were building a server, it would likely be Atom based.
Something like
http://www.newegg....em=N82E16813186184 with a SATA controller and RAID5. Not anywhere near foolproof, but better than RAID0. Again, multiple copies of important files on multiple machines with other backups as needed.
Nicer NAS appliances seem to do everything I do with my current server, but they just don't leave room for growth.
Right now, this is going to be a summer project. I'm going to install a structured wiring panel in my house and run 2 cat6 drops into every room. We've had IPTV installed, and it runs better over ethernet than over coax, so I want to make the switch.
I may go with a mythTV setup in the future. We have a DVR with our current TV service, but it's somewhat limited. If I do go that route, that will be an ongoing project. The structured wiring would be a major plus, though.
I do already have a server closet set aside. I have my gateway and my router set up in there, along with my unused server.