x and h pipes

Well although I've been driving for some 30 years now, I never really got into exhaust work. I see these comments and pictures of x and h pipes and I start wondering what the purpose is. I recently bought a 63 Dart 4 door with a slant 6 225 pushbutton auto. Been thinking about putting in a 318 and going with dual exhaust. What is the reasoning behind using an h or x pipe and is it something I should think about when I swap engines? Does it make a difference if I swap trannies to a 4 speed on what exhaust I can/should use?