an idea for games like this: variable resolution
 

all right there are two kind of things planes and the rest of things

the authentic resolution is 1600*1600 but this is just used for planes for all the rest there should be a simplification algorithm where each 4*4 pixels is simplified into 1 pixel which are actually 16 which are identical in colour, so the graphic card hardly makes effort rendering them

on this way you have planes at a resolution of 1600*1600 and the terrain at 800*800 pixels

i hope you understand and i can see this in the future :)