As you can see from my signature, I have a mediocre system. However, it runs FSX fine if I accept some limitations. I use only enough autogen to keep it interesting, keep simple clouds, fly mostly GA aircraft, and accept that a airliner at JFK is going to run with "jerky" movement. I'm reasonably pleased because my system matches what I want to do. If you want to fly add-on big iron to and from major cities, take others advice and get a more powerful system.
Intel I5-650 @ 3.2 ghz OC to 3.95, 8 gb DDR3 2200 RAM,ASUS P7P55D PRO,
EVGA GTS 250 1gb, CH Eclipse yoke, CH pedals, Track IR, GoFlight GF-46 GF-T8,