Sorry to tell truths about RAM's
FSX is 32bit operating program so it realy can't never use more than 3GB of RAM instead 64bit programs they can use more RAM
Well I'm no Computer guru.
butt better explained in here
http://www.fsbreak.net/podcast/2013/1/16/fsbreak-121-flightsimcon-2013-and-x-plane-10-news.html
if you listen news from X-Plane 10 there is few things about 32 bit and 64bit programs.
Sorry for typing errors