Observations from my install/setup:
With similar settings, I'm getting similar frame rates, but FSX looks nicer at those similar settings.
With everything but weather maxed out, and Clear Skies chosen, it runs (frame rates feel better than 5fps...) but only just. As long as I point it over water rather than land it ticks over nicely

And having never been to Princess Julianna, think I'll be flying there tomorrow in my FS2K4 setup... Did the visual approach on the second attempt (tried it last night, but that was before I reset all the controllers, so my joystick and yoke were fighting for control of the ailerons and elevators, making aircraft control "interesting"), but for some reason the brakes are locked on in FSX (made roll out easy though...)