Personally, I've helped too many customers through really bad data loss incidents with every kind of NAS on the market to fully trust any of them. Data recovery services charge obscene amounts of money for straight NTFS data... Raid recovery, when even possible, makes obscene look downright PG. Exotic non-standard RAID formats make things even worse. Plus, Raids only protect from hard drive failures... Ever consider what happens if the Raid hardware itself fails? SOMETIMES, you can get identical hardware (if it's still available) and get them up and running again... and sometimes that doesn't work put as expected and the new hardware automatically reformats the "new" (to it) drives. Just too much potential for hinkyness.
Hard drive prices have indeed gone up lately, and will go down eventually, but meanwhile it's not the end of the world to pay an extra $50 (less, with some legwork) for a 2 tb drive.
So my preferred solution is to simply buy plain, no frills USB hard drives (USB2 has plenty of bandwidth to supply multiple full-bandwidth HD video streams so USB3, while great, isn't really needed for this application). The fancier the hard drive, the less interested I am. My current favorites is the Western Digital Elements 2tb hard drive. Add a USB hub for $10 if you need more ports. (again, cheap and simple to replace if needed)
The key is to buy in pairs. With two drives and either a script or simple software (like robocopy or second copy), you can mirror the data with a simple file level copy every night. If one drive dies, you still have all your data on the second drive... Just like in a raid, and no hassle, and concerns about hardware failures since its just a simple USB drive. (should the drive electronics fail, just rip out the hard drive and plug directly into the computer, or into any cheap external enclosure) Dead silent, too, and easy to plug/unplug as needed should I decide to save energy... Although generally I just leave them on (spun down, in the case of the copy drives).
Two terabyte drives hold LOTS of data. Expensive Raid setups make bigger "virtual" drives, but why is that so important? The front end software manages the actual data files anyway, who cares which drive the files are on?
I use iTunes to stream music (and videos) from 2 two terabyte drives (each with an identical clone that is updated nightly). If one drive dies, easy enough to pick up another and copy the data back from the copy drive (or to simply replace the copy drive). It's not a hot spare situation like a Raid, but this is music and movies, not mission-critical data for a corporation. Need offsite backup? Another drive is easy to add, and rotate offsite.
I like iTunes, as it does a great job of juggling lots of music and videos in an easy to use format, streams to AppleTVs and other computers running iTunes (great TV interface, too). It works with MP3s, purchased or ripped M4As (AAC-based format native to iTunes and the iTunes store), WMAs (of which I have about 6), and ALAC (Apple lossless, similar to FLAC, both true lossless formats), all but WMA work natively with iPods and iPhones and iPads as well. It manages ripping very well, too. Oh, and it has some seriously kick-butt vizualizers built in, too.
It's a bit resource heavy, certainly, but with today's computers is this really a serious consideration anymore? (unless you're trying to game on the same system and need every spec of performance?)
Raid setups are a GREAT idea, in theory. They add significant expense (I'd rather spend that money on drives, not enclosures), however, and significantly complicate all but the most simple recovery. I think the KISS policy has a lot to offer for systems that are already overly-complex.
That's my opinion anyway. Lots of people have systems that work well for them, and even recover gracefully from problems, like Tom's system above. (I've seen the same systems fail miserably, unfortunately) So it all depends on where you want to roll the dice... I choose the cheaper and simpler option.
YMMV