I think you need to look at Windows Home Sever. It beats the pants off of any NAS that I'm aware of. Here's a little blurb on it that I wrote some time ago:
Windows Home Server is probably Microsoft’s best-kept secret, and that’s a shame. For if you run a household full of Windows computers, you need this product. With the addition of a Windows Home Server machine - which can be built from an old, unused PC – you will no longer have to worry about whether your precious data is backed up – not to mention that of your wife or kids. Windows Home Server did not first appear in 2009, but it did add features that made it even more reliable in 2009.
There are all types backup options available these days, and most of them fail in one respect or another. They’re either too expensive, too inconvenient, or too difficult to use. Windows Home Server is none of these, and it’s available now.