Best tweak ever. I run KSP 1.0.2 32 bits on Win7 64 bits with an i5-4460 and a GTX 760. With Dx, the game was using 2.6Go(on the main menu just after the loading). With OpenGL, I'm around 1.6Go in the same conditions. That 1Go gap seems to stay the same in other situations(loading a savegame, flying for the same amount of time in the same places, building a new rocket in the VAB). I had some issues with fullscreen, but -popupwindow solved them all, just playing in windowed mode should also do the trick of course.