I downloaded the demo of flightsim X and my benchmarks were horible no matter what setting i was on. I was like ten fps on the lowest setting and about 6 fps on the higest setting. I'm not sure why im not able to play Flightsim X at 6 fps but at times half-life 2 episode one is at +200 frames. see my system specs to the right
Your system should be able to play it well, so first I'd check your graphics drivers are properly installed. Then, if that still doesn't work, and you've eliminated software issues, you may want to think about upgrading your CPU as its the bottleneck of the system.