you should really get a quadcore cpu, because FSX really loves extra cores..(and OC it). It isn't as demanding on the graphics card though, so you dont need the most expensive card out there, something like 4800 series or 5700 series should work just fine.
I have read that FSX mainly scales will CPU cores and speed, ignoring the GPU to the point of needing a fast quadcore to keep it steady regardless of the rest of your specs. This is not to say that your IGP won't be a bottleneck (because it will) but without a fast quadcore, you might as well stay with the IGP, because a new videocard won't make things all that much better.
While some of the above suggestions are good ones, you are probably not looking for a full system upgrade.. While your processor may limit the full potential of the game, getting a graphic card as you suggested will offer some improvement.
Please tell us what kind of power supply is in your computer?
Also, what kind of budget do you have available for this upgrade?
Finally, do you have time/interest in learning how to overclocking your processor?