Or if you have an old system You could try
FreeNAS Though probably best if your old system is small, quiet and slimline if you don't have anywhere out of sight to hide it.
Most of the time it will work out cheaper with freeNAS and I would also say that most of the time its also a fair bit faster. Also features BitTorrent, uPnP, iTunes server as well as a lot of others.
edit: I run my FreeNAS on a HP D530 USDT, P4 3GHZ HT, 512MB RAM with 1x2TB drive and 1x1TB drive linked in via a dual SATA USB enclosure. RAM seems to be stuck between 30% - 33% usage and I have never seen CPU go above 5% on each thread and I run BitTorrent on mine non-stop as well as serving blu-ray images to my media centre.