Why not just build an Atom system? The power use is probably comparable to any NAS you can find anyway.
If you're going low-scale, Atom with one boot drive and one storage drive, both 5400RPM, is plenty, and shouldn't use much power. Not to mention it'd be quite a bit cheaper (Atom+board is less than $100, case less than $50, RAM around $50 for 1GB), unless you want to buy a Windows key with it. Not to mention it's easier to work with a system that has some kind of graphical interface.
And if you want to up your torrenting, you can add more drives and throw everything into a RAID array (or not, whatever). You just need a PCI RAID card.