I've read on many forums that you are supposed to lock the frame rates around 35. This way your PC can use it's resources in other areas.
I have mine locked at 32
32. What is the frame rate lock in the hardware display settings and how to use it?
This option is used to cap the fps in FS9 so resources are spared for other tasks (texture rendering, GPS, traffic etc...) than pumping as many frame rates as the system can. Make a few flights with your preferred settings and unlimited frame rates and note the average fps you can achieve, then lock your fps to an amount slightly below that average to keep a smooth flight and best image quality. A nice tool to log average frame rates is FRAPS from http://www.fraps.com