I am using a Matrox TripleHead2Go to provide the spot plane or out the cockpit window views for 3 monitors, which gives a nice panoramic view.  At KSEA with four monitors the virtual size is about 3,100,000K.  If I turn off the fourth monitor, used for the instrument panel, the virtual size drops to about 2,900,000K.  In this case I use the buttons on the CH Products yoke to switch to the instrument panel view as needed.  I think your theory may be right in that with enough RAM FSX can use the full 4GB.  Interesting!

I am using ORBX scenery for the entire region - PNW and Northern California for this KSEA to KSFO flight.

Jim Kaye