A cheap NAS made from a Raspberry Pi model B (the Ethernet one) and an off-the-shelf USB external HDD.
At around $50, might be cheaper to go with this. Unless you are implementing the NAS just because it can be done :-)
I think more interesting applications are in those field in which you can take advantage of the video HDMI port and the IO ports..
[1] http://ahsoftware.de/dockstar/