I have a similar setup as the original poster. In fact, same graphics card.
I did not adjust the graphic's setting that the game recommended, but, my resolution is only 1600 X 900, I know others crank it up. If yours is set to a high setting, try lowering it.
My frame rate, flying around Jacksonville, FL is around 30-40 FPS. Now, as I get closer to the city, it will freeze for about 4-5 seconds, guessing it is loading things, but then it smooths out
Also, I will throw this in, but, is your GPU running the latest drivers? Crazy question I know, but sometimes we may over look something obvious :) (happens to me every day, lol)