lacrimosa09,
It's important to understand that unlike gaming titles which are typically GPU intensive, FSX is an extremely CPU intensive simulation. It will use all the CPU horsepower you can possibly afford to throw at it. Higher clock speed means higher frame rates. Also, FSX became multithreaded with Service Pack 1. Unlike many gaming titles, Hyperthreading improves frame rates on FSX, so performance is better with an i7 than an i5.
I run FSX and X-Plane at 4.7GHz on an i7 4770K, and it's
still not possible to max out the settings without FSX becoming a slide show. The key to the best frame rates in FSX is a highly overclocked i7 with high-end cooling, and a solid understanding of how to tweak the settings and the FSX.cfg file to get the best balance between frame rates and image quality.
If you don't want to overclock, the i7 4790K has a base clock of 4.0GHz and Tubo's to 4.4. The i7 6700K also has a base clock of 4.0, but only Turbo's to 4.2. Nevertheless, since 6th Generation processors are about 5% faster per clock than 4th Generation processors, both of these CPU's are fairly evenly matched for FSX. If you do overclock, then I recommend the i7 6700K, which runs cooler.
As for GPU's, FSX runs better on nVidia cards, but since it's not GPU intensive, a mid-range card works fine. The GTX 1060 was just released yesterday, but the partner cards aren't yet available. If you need to build now, then the GTX 960 would still be a good choice. If you want to save a little money, then the GTX 950 would be your best choice.
CT