I currently use Infuse to play movies on a HDD connected to an AEBS. Although router dependent, your scenario is easily doable. I recommend Infuse over Plex and VLC. I have tried both. Plex is good if you want to stream to different devices (iPad, iPhone, and want to stream while not local). VLC for the aTV4 is a mess right now, especial from an audio standpoint.
Infuse plays most anything without needing a server, like Plex) running on a Mac or PC. They also have great HD audio support. It's $10 (I think) for the Pro version which gives the DTS and HD audio support. But well worth the price. If you don't need remote streaming, Infuse is the way to go, IMHO.