Stock standard, FSX will not run well on your system. Fortunatly guys out there have come up with a few tweaks that improve performance a lot.
These are the settings I have painstakingly (is that a word?) found to work best on my system after weeks of work. I have:
Intel Core2 Duo E6400 @ 3200Mhz
2GB Corsair DDR2-800
If you have a better system than this, obviously crank up the settings higher. If you have a slower computer, I would recommend reducing the Autogen Density as this does effect performance a lot.
Firstly, visit this site and use all tweaks there, apart from:
Do not use the replacement Autogen file. This removes most of the tree models from the game, leaving only a few. I did this without backing up my old file, so had to reinstall to get my nice trees back. It does improve performance though, so do it if you really need to.
Dont delete this file or use the replacement. Doesn't have a large effect.
Disregard that guys sliders, mine are better:
Taxiway texture replacement: