Well my advice is probably not what you want to hear, but...

If you want a NAS, build a NAS.

If you want an HTPC, build an HTPC.


You can make an HTPC with plenty of storage, but if you want that kind of redundancy you'll need fans, swap bays, etc... and it won't be running an OS that is friendly to being a NAS. So you won't have a quiet machine (using 850w+ PSU's will be absolutely required to run a bunch of disks AND a capable graphics card, fans), you won't have the best OS for a NAS (Windows really does suck in this regard, it's just not made for it), and if you did run a Windows OS that is made for it (Like Windows Home Server), you wouldn't be able to use it as an HTPC, it would lack all the kick butt home media stuff.

If you build a NAS, I would do some reading over on Freenas.org about a stand-alone NAS machine, one that you could run a cable to, and shove in a closet somewhere. The benefit of going with freenas is the storage is very scalable... and the platform is very robust as well.