Remember, though, that FreeNAS is still not stable software. I had trouble with it not remembering settings and a few other things. Great concept, but I can't wait for them to iron out a few wrinkles.
After reading horrible reviews about set-top NAS boxes, I built the client a PC and threw Debian and samba on it. I use Acronis True Image Home on their PCs, and have it set to use no more than 100GB of share storage for backups. It worked great for a while, but then there was a power issue that zapped the power supply and got most of the components in the machine, and now I'm trying to get it all working right again.
In retrospect, if I had it to do all over again, I would have gone with a cheap Dell server with a warranty or something rather than building the machine myself. I believe the extremely cheap components I used were a contributing factor in the failure. Which leads me to another lesson learned: always talk to the customer about their price expectations! Don't always assume they want it as cheap as possible! In my case, I talked the customer out of manually backing up to a USB drive with a few computers into instead using imaging software to make backups to this NAS. Therefore, I assumed they would be shocked at a high sticker for the PC used as a NAS, when comparing it to the cost of a USB drive. Later, they made a comment about cost along the lines of, "Don't worry about how much it's going to cost, let's just get it working." That kind of threw me for a loop.
On the plus side, such a configuration can be used to add value later, and the customer has asked for a light website that would provide their customers with the ability to upload files that are too big for email. The server should be able to handle that small increase in activity just fine.
Keep in mind that most of the small business/home office sites don't have incredibly stable power infrastructure. Make sure that the NAS has a UPS, and a daemon running to monitor it and shut down if necessary. In my case, it's an APC 800VA UPS and the NAS server runs apcupsd.