A friend of mine lent me the DSM-320 (it’s coming back soon, Patrick) and I evaluated it to decide to buy the ($229) DSM-520. Both boxes are UPnP AV streaming devices that let you stream audio, video, etc. from a PC.
Here’s the differences (all in favor of the DSM-520):
Supports WPA encryption on wireless
Supports high definition component and HDMI outputs