since flying with Tileproxy, the performance has been kicked up quite a lot!
Of course a the vegetation and many buildings get lost, but that's okay for me go get a photorealistic satellite terrain with a much better performance.
But if you internet connection speed is too low, so that the loading of the terrain is not fast enough (i'm using DSL 16.000 kbps, which works ok), or prefer the original FSx terrain from other reasons, you might also look here:http://www.flightsimworld.com/forums/index.php?showtopic=92413
Or just google "fsx autogen"
FSX is definitely CPU-limited. It makes practically no difference in performance between flying with my GeForce 7950 GT or my GTX 260.