Yup it's just a line you add to your fsx.cfg. Google HIGHMEMFIX for how to apply it. It won't help the OOM issue but it helps other problems with FSX memory management. It's basically the one "tweak" you can't do without.
Another thing you can do is install the app Process Explorer from microsoft, it will let you monitor FSX's VAS (virtual address space) usage so you can see when your approaching the limit. You can also use it to see how particular addons, slider adjustments, graphics settings, scenery, etc effects the VAS size, to learn how best to stay below the limit with your system.