Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
You never mentioned what is your drop dead amount is to spend. Old Intel i7 920 technology is pretty much dead and buried as is FX6300. The new Haswell i7 and i5 CPUs are by far faster processors than the AMD FX6300 or even the newer FX9590 and you can pick them up for just a little more money at newegg.com, amazon.com, or microcenter.com. You can run FSX on AMD just fine if you are willing to overclock! However P3d and X-Plane 10.3 will lag considerably. It is not GHz alone that determines the fastest chip. Even at the so-called AMD 5 GHz range it is still slower than the Haswell i5 and i7 stock turbo range. Also the Nvidia video card performs much better with Intel. If your choice is AMD you might want to consider an R9 series AMD video card for best performance.
If your interest is only FSX AMD oc'd to 4.2GHz or better is just fine. I still recommend the AMD R9 series video cards for it and they have a new catalyst software that works great. But the cost for AMD FX anything vs Intel i5 Haswell is almost the same on sale. The i7 Haswell has even more features but does cost more. So depending on your budget if you comparing the FX6300 vs i7 920 you will have to overclock either to get FSX smooth. Neither will run P3d or X-Plane 10.3 at realistic frames per second.
Ted