dont worry about that.
buy the best performance or vfm in the market
there is no thing called as green hard drive or low power
its going to rotate at certain rpm
plus hdd has a very low footpring in overall power consumption
look for other components like smps,processor etc to save your power.
I would agree that HDD power consumption doesn't differ to a level where there would be an immediately noticeable running cost change, however if you are after HDDs that are Eco-conscious and have slightly reduced power footprint then you may want to look at WD Green drives.
Western digital green drives is what you should check out. I would also like to agree with Stradivarius you shouldn't worry about hard drive power consumption. You should consider using other components that are more green and less power consumption such as processor , GPU etc..
Let us know what you choose to do and what parts you are going to use in this build.