Either HIGHMEMFIX=1 or HIGHMEMFIX = 1 will work, provided it's in the [GRAPHICS] section.
However, if your FSX.CFG has OTHER errors, or you added errors by editing it, it will be recreated again from scratch. For example, I see you typed [GRAPHICS ] ( with a space ) in your post, when it should be [GRAPHICS], with no space, and that's clearly an error.
But the whole point is that you don't even have to edit the file manually: haven't you noticed the Addon Manager has a page of tweaks, including the HIGHMEMFIX, so you can just enable it and Save the FSX.CFG ? Of course, the effect will only appear on the next restart.