I think it's probably the forced AA, and maybe the lack of timeslice shadows if you have it off. I'm running a similar rig and get 45-60 FPS constantly at 1920x1080 with everything maxed except for no AA and timeslice on. Honestly, to me the performance hit from AA is not worth it at all at 1920x1080 since I barely even notice any jaggies at that resolution. I know that's personal preference though since some people can't stand the lack AA at any resolution.
