Dave I am in the middle of a gamer build right now for a guy and trying to get it done and tested this weekend. I will get with you Monday and give you the keys to the kingdom so you can decide for yourself what you want to spend and how you want to spend it.
Hard to find but definitely a winner CPU Dave PRB is right! I have OC'd them to 4.4GHZ easily and mated with 16GB of fast ram and this: http://www.amazon.com/EVGA-SuperClocked-Dual-Link-Graphics-02G-P4-2765-KR/dp/B00DHW4HXY/ref=sr_1_1?s=electronics&ie=UTF8&qid=1397950642&sr=1-1 she will easily stay at 45 to 50fps
Whatever else you do, get at least 8GB of RAM and a 64-bit OS. That leaves FSX free to use its maximum memory footprint of around 3GB+ without choking the system by paging the OS off to HD or SSD. (Win7 Ultimate 64-bit with 8GB here)
Since the year 2000 Intel has been the far superior processor for gaming and that substantiated by multiple test sites. However I have seen AMD processors run FSX and P3D very well. It is also a bit less expensive to run the AMD platform, however Intel has become extremely competitive.
I have found the same problem in the following airfields: