The first thing to recognise is that FSX is a 32bit aplication so it is incapable of using more that 3.5gb of RAM howver many memory modules you install on your PC.

I believe, on a 64 Bit OS, that FSX, as a 32 bit application, can address a full 4.0 GB of memory.NONE of that 4 GB addressing space is used by the kernal addressing space in a 64 Bit Operating System. Even on a 32 Bit OS, using the 3 GB switch, and setting applications to address 2.5GB (instead of the default 2 GB), is sufficient increase in addressing space for FSX and PMDG 737NGX to run quite happily, without causing an OOM due to memory addressing limitations,

Forget Windows 8 and get Vista 64-Bit. Works great for me. Yeah Vista is a resource hog but hey its Stable and FSX loves it. Obviously it don't like Windows 7 as much or people wouldn't have to jump through so many hoops to get it to work right. I may try Windows 8 someday around SP1, but why in the world would I trade a finally tuned perfectly running Vista for Windows 7?