i7 4790K and a single GTX-770.
Unlike most "gaming" titles, FSX and X-Plane are "simulations" which are very heavily CPU bound, but very lightly GPU bound. As such, neither FSX nor X-Plane benefit from CF / SLI or high-end graphics horsepower. Conversely, since multiple cards require more CPU interrupts, frame rate can actually decrease slightly.
As I've explained in many threads, frame rate scales with the number of CPU cores and clock rate, so a high overclock produces the best frame rates. Further, since these simulations perform better with nVidia drivers, a mid-range nVidia based graphics card works well, which is why X-Plane recommends a single GTX 770.
The following link is to X-Plane's website - http://www.x-plane.com/store/hardware/ -
and to their partner's website, XForce PC, who builds their PC's - http://xforcepc.com/store/index.php/computers.html?mode...
I run FSX and X-Plane frequently, so I've built and configured my rig (below in my signature) to accommodate them. I would recommend a single 770 and the new i7 4790K rather than the i5 4690K, as Hyperthreading improves minimum frame rates. Your rig will run FSX best with this configuration.
Hope this helps,
CT