The only advice I have is keep the files < 4GB.  The DNS-323 UPnP catalog has a 32-bit bug in the file scanning routine.  Files > 4GB will show as the remainder of Filesize/4GB.  This causes the files > 4GB to stop playing well short of the full length.  For OTA HD video content, this is about 20 minutes.  Quite a nusance.  These same files play fine from a UPnP server running on a PC or Mac reading them from the DNS-323.  If you have alot of files > 4GB, you may want to consider using something like Windows Media player or the Nero software that came with the DNS-510.  (if you don't have anything better)  Hopefully D-Link will do something about this soon.  It would be interesting to test for this bug on the new DNS-321 and DNS-343 boxes.  Chances are the firmware is similar.
Cheers,
BSPvette
PS: Use a wired connection if you are playing HD content!