But i dont use a single add-on except REX, FSDT JFK and PMDG 737. Nothing else in this area. So i cant really belive just because of these 3 add-ons my FSX is running out of resources.
And you call this "nothing else" ? You are running one of the most resource intensive airplane, in one of the most densest areas of the FSX world which is very demanding even in default, with a very high resolution textures addon, plus a detailed airport, it's fairly normal you will run into problems if you keep your settings too high, especially the Scenery Range. And, which kind of AI and AI settings are you using ? They count too...
Even if you have 8GB, FSX won't use more than 4 of it, because it's a 32 bit app, and 32 bit apps in a 64 bit OS can't use more than 4GB in any case.
JFK scenery doesn't have anything to do with this, nor any of the other addons you mentioned, it's the *combination* of them that doesn't work at JFK with *your* settings, because the JFK area is already very dense even without adding any addons.
It's fairly sure that, if you try to do the same with a default airplane, it will not crash, or if you lower your settings, like the message is suggesting. There are plenty of messages on all websites reporting Out of memory problems with FSX and some complex airplanes in every places, not just JFK. If you haven't experienced it anywhere else, except JFK, it doesn't mean JFK is a problem, it only means your system is enough to handle other areas with your combination of addons and settings, but not the JFK area with your combination of addons and settings, if you had a lesser system, you would have notice it at other places too, so you wouldn't be saying that it must be JFK.