The tweak that NineLives is pointing you to is the ticket. FSX is a resource hog, and even with a high end machine, you still need to get your settings right.

I'm running an i7 960, 12GB RAM, 2 GTX580 cards, blah, blah, blah -- I STILL had to get my settings right. After the Venetubo tweak, it's smooth as glass at 1920x1200 with in-game settings mostly maxed out.