Ahh crap sorry... I was a bit too quick on the reading.

Have you done a Control-R during gameplay to see your framerates? Your graphics card seems adequate for everything. I'm guessing you've got the graphics cranked up. Crank them down and see what happens.

You can also see if your CPU is the bottleneck (although I doubt this is the case), by running task manager while playing. Yes, the very act of running task manager will use CPU cycles, but it will give you a somewhat close view of things.

Do you have the same problem in Shatt or other cities?