Actually, I have no idea when exactly it switches on enemy planes. I am sure, however, that it does switch in F3 on bombers somewhere around that distance. And when I say bombers I mean Ju88 which may have a kind of weird bug, with sometimes only parts of the skin switching.
But maybe 1cane is attacking ju88 and sees the same from the outside.
3GB doesn't seem like a lot for a video card these days.