This is how a watch video files (that I've downloaded to my NAS) on my HD TV via my Roku box. This turns your DNS-321 or DNS-323 into a streaming video server that a Roku box can see. Note that there is no PC in the loop during streaming/playback -- the video goes directly from the NAS to the Roku box (that's connected via HDMI to the TV).
See my post at http://forums.roku.com/viewtopic.php?f=28&t=25955&p=174513#p174513