I don't know much about FSX, in fact I have never played it. But if I had to make a recommendation, I'd base it around around the recommended system specifications. It doesn't sound like FSX uses the specifications that was suggested by Rick, and I highly doubt that the program would be capable of utilizing 12 CPU cores. I've looked at several websites, it sounds like you could get by on the 500-1000 dollar range and have a pretty outstanding machine for running that game. Regardless, the main thing I'd like to suggest is looking into using a projector with a half oval shaped screen.
There is some sort of trend aimed towards eyefinity, the idea is nice but I'd much rather use a projection. If you want big, go with a projector, there is no breaks in the view.