Since I put 32GB of RAM in my computer, FSX has run better. So my theory is this:

I think it all comes down to how much RAM Windows allows FSX to use. Once you have a certain amount of RAM on your computer available, FSX eventually gets the full 4GB of RAM it requires. This seems to be somewhere between 16GB and 32GB.

I have not had an OOM for a very long time now, and definitely not since I installed the 32GB of RAM. But then again, I no longer have any 3rd party scenery installed either (I uninstalled it some time after I put the 32GB of RAM in my computer), and when I did, it was only Obyx FTX (AU, NZNI, NZSI, and a lot of Aussie airports).

Edit. My system isn't overclocked, and only I only have a single 24" monitor (as per my specs in my signature).