Just dived into this subject not to long ago with other user. Its a good read with loads of information (link to NAS build thread
Just a recap without reading it all:
I basically pointed out two off the shelf NAS solutions, the Synology DS411+11
for $650 or the QNAP TS 489 ProII
for $960. The major down side with using an off the shelf NAS box is price and some boxes lack some features (which you would have to pay for on higher end models), but the best thing about them are their simplicity, ease of setup/use, actually pretty feature rich, low power, and you get simple raid 5/6 solution.
But if you are a savvy user, you can build your own NAS box solution using regular PC components and honestly, doesn't require a lot of cpu processing power (reason why most NAS use low power, cheap Atom cpu's). You can build one for very cheap and either use a copy of Windows Home Server (which actually is pretty cheap and easy to setup) or go with a more tech savvy approach and use opensource software like FreeNAS OS
For a self home build, this is what I'd look at doing:
CPU: AMD Sempron 145 2.8ghz
Mobo: ECS AM3 AMD 770 atx
RAM: Mushkin Enhanced Essentials 4gb (2x2gb) DDR3 1333 cas9 1.5v
Case: NZXT Source 210 Elite White ATX mid tower
Case fans: 4x Rosewill 120mm case fan
$2.83each = $11.32
GPU: ASUS 5450 512MB pcie
You could upgrade the cpu to a higher end x2,x3 or x4 chip just for "future proof" and if you decide to do any transcoding on the server side for viewers. Since you can't really get new x2 or x3's, then a "cheap" amd x4 631
is $85. Another option is to find a used chip on ebay, if you are willing to save a few bucks.
Added a cheap mobo and some cheap 4gb 1333 ram cause well... that's more than enough. The board is def a toss away cheapy oem build that will be fine for this application. As for the case, this is a decent NZXT box that has 8 internal spots for hard drives and slots for decent air cooling (reason why I added 4 120mm fans).
As for hard drives, this is what you can do; grab two or three of these:Seagate Barracuda 2tb 3.5" 7200rpm hdd
$150 eachHitachi Deskstar 7k3000 2tb 7200rpm
I only mention the Hitachi drive only because lots of people have said that the cheaper seagate have high DOA fail rate; so probably stick with the Hitachi. If you got a small toss away sata drive, I'd probably use it for just the OS drive. Don't really need a big one. Now the most limiting feature of this build is that the cheap mobo doesn't support raid 5, so if you want that feature, you have to get a proper raid card.
For raid, you can get the cheaper HighPoint 2640x4 pcie x4 card
for $96 that'll allow up to 4 sataII drives in a raid setup, but you could go with the HighPoint 2720sgl pcie x8 card
for $160 that uses two SAS connectors that allows up to 8 sataIII devices, but you have to use the expensive SAS to SATA cables
for $31 each.