ant_1984

Ben, I came from a system virtually identical to what you have now with the exception of having a GTX 680 and some 2133Mhz RAM, but my 2600K was also at 4.9Ghz.
 
I now have the system you can see in my profile. I have been very happy with my upgrade and performance seems overall smoother, FPS are better in FSX but not earth shattering. Smoothness for me is where I 've seen the improvement.
 
Keep in mind though that I also went from a 1080P monitor to a 1440P monitor, so my performance increases would have been more defined if I was still on the lower resolution. That being said, I could never go back to the lower resolution and I am getting better performance now at 1440P than I was on my old system at 1080P.
 
CPU/RAM I think you've made the right choices. Just try to get the RAM with the lowest latency possible. You would also do just fine with some 2400Mhz sticks.
 
GPU, coming from a 660ti I wouldn't go below a 780, specially if you plan on going for a higher res/multi monitor setup. That's my opinion only though. Buying at the end of the year may serve you well. The 780ti is due mid November and while pricing/specs haven't formally been released, it's likely the 780 will drop in price and the 780ti will fill it's price point. We'll obviously have to wait and see though.
 
Don't expect to see a MASSIVE improvement FSX wise (don't get me wrong there will be some improvements), but any other games you use will likely see a boost in performance from the GPU upgrade alone.