Sharrow is right about the Intel processor, but that dosen't mean you can't upgrade to a better AMD processor. I recently built a new system for a friend using a Gigabyte motherboard with a AM3 socket and a 965 Phenom II quad core. It was more than suitable for FSX, even with all scenery sliders maxed. You would still need to be somewhat conservative with aircraft traffic settings, but put the other traffic settings to max and forget about it. The good news is that a number of the quality AM2 and AM2+ motherboards will run the AM3 Phenom IIs pretty well, though the BIOS might need to be updated.
If you can tell me what motherboard, or computer, you are currently using, I would be glad to do a little research to determine what might be a good processor upgrade.
FSX is very CPU dependent, so this would be the best, single, improvement you could make.
See ya, Milt
If it ain't broke..., fix it any damn way!!!