I just hung a second Samsung on the vid card (it has 2 DVI connectors), made no other changes to FSX, had 20FPS at max settings except "special objects" at dense and traffics at 50%.
Still have 20FPS on main screen with GPS + radio stack on secondary monitor.
Haven't played for couple of days, the dirt strip I fly out of was sandy colored with brown patches, but is now green with brown.
Does FSX change terrain palette seasonally?
Yes. Check your weather settings. It is designed to change along with the date/time