When you are in FSX, press shift+z twice. This will display your fps (frames per second) rate in the upper left corner.
Go into your display settings and do this:
1) turn off all traffic and vehicles (this is so you can tune your scenery settings without the traffic affecting your fps)
2) experiment with different display and scenery settings while you watch your fps. The 2 settings which will have the most impact are Scenery Density and Autogen Density. You can also experiment with aircraft shadows on/off, detailed clouds vs. simple clouds, level of texture detail (low to very-high), etc..
3) you can turn off the fps display by pressing shift+z one or two more times
Tips:
Airport vehicles are a HUGE fps killer for some reason. Even worse than airliner traffic.
Water effects look good at or above the "2x low" setting. If you have to keep it lower its best to just have no water effects in my opinion.
Once you are happy with your settings you can save them as a custom display setting (like the fsx presets 'low,medum,high, etc.'). You can save more than one custom display setting, which is handy because different areas have different effects on fps. In other words you can save one custom setting for flying in dense city areas, and another custom setting for flying in rural areas.