I've heard that the patches to FSX optimize it to use all 4 cores of a CPU pretty effectively.
So far FSX is capable of using up to 32 cores but had the possibility in the future of using 256 cores.
Read the posts by CompuTronix as he is the FSX expert at Toms Hardware forums
He states that there is an astonishing 80% improvement with using 4 cores over a dual core CPU which would be very efficienthttp://www.tomshardware.com/forum/247975-28-quandry
If you're going to upgrade your CPU at this time it would be better to choose a Phenom II X4 955 BE or a Phenom II X4 945 over last years older Phenom X4 9750 if you have a compatible AM2+ motherboard with BIOS updates that can take these CPU's.The Phenom II X4 955 BE is about on par with Intel's Core 2 Quad 9550 CPU.
By the way what is your motherboard make and model? if you're running an AMD Athlon X2 6000+ CPU it has to be either a AM2 or AM2+ motherboard.
However the best CPU to choose for high end FSX performance would be an Intel Core i7 CPU
I think that CompuTronix's new system is a Core i7 920 oc'd to 4 Ghz.
Son of a U.S.A.A.F. 15th Air Force 463rd bomb group 772nd squadron B17 pilot.
Video of my dad on a B-17(So you can kind of meet him-he passed away in 2004)http://www.youtube.com/watch?v=bGOP53TkGtA