Perhaps this isn't an answer to your question directly, but my own experience has been that typical NAS hardware costs about the same as a regular PC. The only reason you might get NAS vs a PC is its lower physical profile.
We evaluated the NAS vs PC question quite a bit and eventually ended up with PC, because it has significantly more flexibility for us. We could install any OS we wanted and in this case, getting the box(es) on our Windows Domain was much easier than with a NAS. The PC was also better-performing than ALL the different NAS hardware we tested. We can also add as many drives as we want, and even had eSATA capability.
With PC's able to be built to very low-power profiles, it would be easy to build a dual-core, multi-drive PC-NAS that consumes less than 60 watts or so under disk-intense conditions.
Just some thoughts.
Additional point: File serving and backup should be two different functions. My recommendation would be to have one or more external hard drives connected through eSATA or USB 2.0 for backup purposes, and create a process on the fileserver to automatically backup / sync the backup drive with your data folders.
The fileserver acts as a fast repository to serve / host files from. If the fileserver goes down, you still have easy access to your data by connecting it to any working box with USB or eSATA. The external drive is also useful when you go on trips - making it easy to take your data with you.
I would recommend this over any RAID configurations for most scenarios, because when RAID fails it is often difficult to recover or rebuild. Given the plummeting costs of storage, RAID is just not worth the hassle anymore.