Not really knowing what your budget is kinda hampers what advice to give. What I can say is if you have the money, i would scrap your current system. I had a pc that I built that was running a AMD phenom ii 965 BE cpu that I overclocked to 3.8GHz. I decided to switch to a intel based system. I was working on a budget so what I did was I sold off the computer case, CPU, Motherboard, my H70 liquid cooler. I kept my PSU, hard drive, memory sticks, cd rom drive, and GPU. I sold all the old parts for 500. With that i purchased a i5 2500k, new case, new motherboard, and a new H80 liquid cooler. The great think about the 2500k is the k is a unlocked cpu so it can be overclocked. I will be overclocking to 4.5GHz. My old pc could run FSX with a lot of add on stuff at med-high to high settings and I managed 17-20 fps on the ground and 30fps locked. All the new parts cost me 500.08.
In your case depending on how much you have saved up, I would sell your current system and build a new tower, or keep some parts such as you hhd, cd rom drive. If you did that and assuming you have a windows 7 64 bit disk, you could build something like this. Even in its current config it would run fsx very well and if you are into overclocking you could overclock it and easily get high settings.
NEW: Windows 7 64bit, Corsair Vengeance DDR3 Ram 8GB, GeForce GTS 450 1GB, Gigabyte GA-Z68X-UD3H-B3 motherboard, Intel i5 2500k 3.3GHz (OC 4.5GHz), Corsair H80 liquid cooler, Coolermaster Elite 430 case.