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Normal HDDs consume about 7-8W meaning you need airflow to keep the drives from building up heat. A WD Green drive will consume half of the power, removing any need for cooling, having less heat build up inside your case and also take care of a lower energy bill. Their sequential speeds are not very different from 7200rpm: about 100MB/s max. And since it'll be used for mass-storage it doesnt need the higher IOps performance of 7200rpm HDDs. Plus they are generally very cheap if you calculate the price per gigabyte.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136317&Tpk=WD10EADS
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136317&Tpk=WD10EADS