AO really isn't optimized to run well. I had this turned on with my Radeon 6970 and I lose 15 fps from it. Turn it off and I run 60 fps everywhere but crowded Ul'dah. I can get away with running Depth of Field, but AO has been a problem for a long time. That and don't run your general graphics settings over 8. I think you could have a 6GB video card with some killer settings, but just the way the game is currently coded you'll never be able to run it at max settings and still get max fps.