I have a ready duo and it works very well with 1080p (mkv), and of course 720p. I have a wire network, i’m not using atv with wifi to play movies on my readyduo.
I hope it will help you in your choice. But i also have the dns-320, and i prefer readynas because it has a better quality (metal vs plastic), and you can install many plugins. And you have a hot swap with 2 racks on readynas. However, i have not tested streaming for dns-320, sorry, and i gave it to a friend so i can’t test that.
I use a DNS-320 but I dont use the streaming on it. I use the SMB sharing and add a share to ATV2. I have had no problems out of it this way. I have it connected via wire to my router while the ATV2 uses wifi. I have 6 ATV2’s all connecting to the DNS-320 with upto 4 running at any given time.
I did use the streaming many years ago but back then it didnt even support mkv files and havent had a reason to try sense.
I have the Ready NAS duo. It was a real pain to set up but once working it is great. No problems streaming etc. I like the fact I can add a further disk etc.