I just received my bto imac 2011 3.4 with the radeon hd 6970m 2gb gpu. I am trying to run portal 2 in native. I cannot. It is not even showing 2569 x 1440 as an option. I can run two worlds 2 in native (looks amazing and smooth as butter by the way) Anyone else having the same challenge? Any fixes? Any help is greatly appreciated. Thanks in advance.