This was a major concern for me when I wanted to do some data storage.
Ultimately, for low power storage, you want a commercial NAS. There is no question about this. The reason being is that commercial NASes can utilize parts that are much lower powered than most anything you can build on your own. Unlike their hobbyist counter parts, NAS hardware is built primarily with one general task in mind. You won't find sound cards, video cards or anything else that can draw power even when left unused. In fact, in some cases, you can get a non-x86 processor to do most of your computing for you (which is MUCH tougher to get for most folks).
Ultimately, I suspect one of the best low powered NASes you could get would be the Thecus 4100
, since it can power down the drives and uses an Intel XScale processor. Unfortunately, it's bloody expensive and unusually loud.
(FWIW, I own the 5200 Thecus NAS, and though terribly convenient, it's still bloody loud. And I'm reluctant to mod this sucker given how much it cost me).