From what I've heard around the board, it seems to handle 1680 X 1050 fairly well thats what I'm using and it seems great. However, I'm not to sure about the 1900 x 1200.
With the right software it'll do 1080p and DD/DTS without any trouble at all. But the "right software" only exists in the form of XBMC (and maybe VLC).
This has been a number one concern for me, even vlc does not distribute the hd video and DTS among the cores very well and you get hiccups now and then. Its just a matter of software however, who knows how long before it works properly. I believe if you double boot into windows and use wmp11 it works flawlessly.
You would think Apple would be the best at programming applications for OS X, but Quicktime is simply awful at it. I guess they've been too distracted with other things lately. 🙄