it really depends on how much you want to spend and how much manual work you want to do. Personally I would get a motherboard from here:http://www.zotacusa.com/products/motherboards/amd
Make sure it has 4 SATA ports and GigE. Get a case, power supply and ram and you're good to go.
Then I would install linux on it and leave it next to my router. There are many distributions which specialize in NAS (FreeNAS appears to be the most popular).
An underclocked dual core CPU will take very little power and they are quite cheap.
If you are not into building your own systems, the more expensive approach is to buy a pre built nas. They start at about $100 more than this build will cost.