Actually, there is a difference in what appears smooth to the human eye between film and 3D graphics. Film in movies has indivdual frames that have slightly blurred motion, and this allows for smooth vision at 24 fps. However, a computer shows still images with crystal clear edges, and therefore the frame rate must be much higher for the human eye to see the same thing. 60 fps appears much smoother than 30 fps for this reason.
I recommend looking at this page towards the bottom:
http://www.tweakguides.com/Graphics_5.htmlThere is a whole section explaining this topic, and also a cool little program that allows you to see the difference between framrates.
To answer bubbleheads question. I found better results setting my framrates to unlimited, but that was my PC. Yours is very different from mine (intel v. AMD and Nvidia v. ATI), but it should still pefrom quite well in FSX. I suggest try setting unlimited and fly a flight, then set it to something like 30 or 40 fps and try that same flight using the same conditions. Just see what works best.
Mark