The system I would recommend if you are building one new is:
AMD FX-4300, 95W CPU (current cost today is $99.99)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113287
The AMD FX-4130 is $10.00 cheaper but it also consumes more power (125W) even though you will likely never use it at full speed.
By one of the ASUS M5A78L-M motherboards:
http://www.newegg.com/Product/Produ...iption=m5a78l&bop=And&Order=PRICE&PageSize=20
As for the motherboards, I chose the M5A78L-M/USB3 because it has four DIMM slots and a maximum of 32GB RAM where the other two have a maximum of 16GB RAM. These are all uATX however the one I chose is 9.6"x9.6" square, the other two are 9.6"x7.4" so ensure you have the proper case size. In my system I have 16GB RAM and I doubt I will expand it to 32GB but I have that capability should I desire it later.
If you were to purchase my system right now the cost would be:
CPU: $99.99
MB: $59.99 (after $10.00 mail in rebate)
RAM:$95.99 for 8GB, total $191.98 for 16GB
Total: 351.96
RAM:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820139978&Tpk=KVR1333D3E9SK2/8G
Now the good thing here is you could use regular DDR3 RAM to start with, especially if you have some laying around just to get the project off the ground and then upgrade to ECC RAM when you can fined a good sale. The price of this specific RAM went up almost $10.00 since I purchased it 2 months ago.
My advice is to create a listing of a few different items you could buy and be happy with such as the CPU, maybe the FX-4300 will be on sale for $30.00 off one day with a combo of a MB purchase. Scan the internet at a few places like NewEgg and Tiger Direct, there are others. Look for the good sales and then purchase. Buy good parts since you are going to sink a lot of money into hard drives. And by all means purchase a good UPS that have a USB data connection so when you loose power, the UPS will signal the NAS to shutdown so you don't have data corruption.
As to the Haswell question, they have an ultra low power mode, it's like a deep sleep where it draws something like .05 watts of power. This causes problems with many normal power supplies because they may trigger an under voltage condition (I won't get into the details but you could google power supplies and haswell) so you need to purchase a power supply that has been certified to work with a Haswell CPU and that really won't cost too much more but it's something yo must take into account. You also would need to locate a MB with ECC support.
Good luck on your venture.
EDIT: You never specified what your NAS would be doing. The system I recommended should handle a small business (20+ people) or less depending on the drives you install, including encrypting the hard drives as AES is built into the FX CPUs. For a home system, this might be a bit overkill but it's fair price.
Something else about ECC RAM or not, if you're running a home system and you don't care if your data becomes corrupt, then you don't need ECC RAM. However if you are backing up your data from the NAS to other storage, how would you know if the data was corrupted when you backed it up. It's a hairy edge and if you want to ensure those thousands of photos or music files don't become silently corrupt, get ECC RAM.
