Alright I did a mini test for you, I was convinced it was the CPU. So I ran the "Sitka Approach" mission with two setting on rig in sig.
Graphic Settings:
Graphics: Custom (All options maxed at 1680x1050)
Aircarft, Scenery, Weather, Traffic: Ultra High Preset
Setting One: Q9550 @ 6 x 450 = 2.7 Ghz
Loading: CPU usage pretty high during loading all 4 cores used generously hitting a peak of 98% CPU usage.
Gameplay: Pretty high cpu usage hitting a max of 100%. FPS hits low to mid 20's close to the runway and during landing and is around 50-60 in the air
Setting Two: Q9550 @ 8.5 x 450 = 3.825 Ghz.
Loading: CPU usage pretty high during loading all 4 cores used generously hitting a peak of 98% CPU usage. But noticeably shorter loading time.
Gameplay: Pretty high cpu usage hitting a max of 100%. FPS hits low 30's close to the runway and during landing and is around 60-70 in the air
Now this is of course with a quad, FSX eats quads, it needs a lot of CPU power over anything else.