What you want with Microsoft Flight Simulator X is the fastest and most powerful Quad Core CPU that you can afford.
Ideally a Intel Core i7 CPU system that is overclocked or the next best a Intel Core 2 Quad CPU based system that is overclocked.
A AMD Phenom II system would be all right too.
Here is a review of the Core i7 and simulations.Some people contest the results in the Sim HQ forum though.http://www.simhq.com/_technology2/technology_111a.html
here is an older review of last years AMD's Phenom 9850 BE and simulations.http://www.simhq.com/_technology2/technology_125a.html
If you're building a PC specifically to run FSX don't bother with the older dual core CPU's.
This CPU would make perhaps the best choice right now for FSX.http://www.newegg.com/Product/Product.aspx?Item=N82E16819115202