FSX should be good.
I will have to very roughly estimate over 30FPS on low traffic, simple weather, real physics, medium high graphics in city with medium autogen and special building, DX10 with most of the special effect on medium to max.
But again, there is so many settings you can change in FSX, it is really very hard to put a FPS on it.
Don't forget, you need Window for FSX.
And no, I don't think COD is as hard on the hardware as FSX.
By any chance you can have a 1GB 6770m instead of a 512MB. It will help in FSX.