Do you use only ONE VGA card with two 1920 monitors? If so, two pieces of 1920 monitors has roughly ONE 2560 monitor in momory, because 2*1920^2 = 7372800 is roughly close to 2560^2 = 6553600.
But two separate monitors need more work (in VGA momory) than one physical monitor, if my understanding is correct.
I use 2 x 2560x1080 using two high end 3D cards.
