It's always fun to see how many approaches there are to stuff like this.
I just built a new file server this week to replace my aging ReadyNAS. I ended up getting a Newegg bundle which included a SuperMicro Atom-based board and a SuperMicro mid-tower (with 80+ 300W power supply):http://www.newegg.com/Product/Product.a ... 6813182233http://www.newegg.com/Product/Product.a ... 6811152123
That SuperMicro board is a bit expensive, but I wanted something with low power draw, passive cooling and which had a lot of SATA onboard.
The bundle was $199 US which I think is a pretty nice deal. I'm not entirely crazy about the case, but it's not bad. Sans hard disks, it was using about 24W at the mains with FreeBSD (FreeNAS) booted up; I've not yet measured it with the disks in there (4 x 2 TB WD Green drives). I had 4 GB of SODIMMs which I put into it from a broken laptop. The SuperMicro board also have a USB port on the motherboard; the same kind of USB port you'd fine on the front of the case. I put the USB stick with FreeNAS on it and boot from it.
FreeNAS really is a nice setup. I have run it on a smaller Via C7 system and it's been rock solid. FreeNAS on even the crappiest of hardware (like my 1.1 GHz Via C7, passively cooled) is way faster than a two year old ReadyNAS, both in terms of UI responsiveness and network throughput.
For the filesystems, I'm not using RAID or ZFS. Part of the reason is to "keep it simple". I like the idea that I can just take a disk out and use it in another FreeBSD system easily. Some of the storage is network backups, and I don't need redundancy on these, so I don't want to run RAID on that. I have an rsync job setup in FreeNAS that rsyncs data across some disks so that necessary data is copied across disks. It does this in the middle of the night. I like that better than RAID because if I delete a file, it's still rsynced safely elsewhere (until 2am, anyway). I really wanted to use ZFS, but it's still a bit experimental in the current FreeNAS, and reliability is more important to me than the (admittedly cool) features of ZFS. I predict I'll be running ZFS in the next couple of years though...
FreeNAS also can spin down disks when not in use, and those WD Green disks use almost nothing when not spinning. Since FreeNAS runs off of a USB stick, the disks are really only used when data access occurs, so one of the disks only spins up once a week for a network backup to be made to it.
Depending on what you're trying to do, FreeNAS can be a nice choice.