When your simulator crashed with an Out of Memory error you have to add a small tweak, add the line HIGHMEMFIX=1 into the [Graphics] section of your FSX.cfg file.
To find the FSX.cfg file simply open the Windows Start Menu and type %appdata%\Microsoft\FSX into the search field and then press enter.
You can open the FSX.cfg using the default notepad contained in Windows. In the file scroll down until you find the [Graphics] section. Right below [Graphics] add HIGHMEMFIX=1 and save the file.
This fix enables FSX to use some more memory and is a line that was simply forgotten by the Microsoft during the development of FSX. The fix can't solve all memory problems, but it should never miss in any FSX setup. Due to its nature, as a 32bit program FSX will never be able to use more than 4GB memory though, regardless of the memory, you have installed in your computer.
This tweak is NOT needed for P3D or FSX:Steam edition!