the look is all up to you and what you personally like...
I think that the normal spacing is 4-6" between lights on standard strings. Best thing to do is hang up one string, and then a double string across from each other and see what you like best.
This year the "mega" spiral tree that we built only has 32 channels and about 3200 lights, but it still looks pretty nice. On the other hand 1600 lights could look just as nice depending on your build, height, and programming.