Guys, I think there is a very strong possibility halo 5 will be on the x720, reason is, the team at 343i stated numberous times that "Halo 4 will put the x360 to it's graphical limits", and every time a new halo game comes out, the graphics improve (excluding ODST). That means 343i will need more firepower for halo 5 to run smoothly, so it would make sense to make it on the x720.
Oh yes guys, the x720 will use AMD chips, but microsoft stated something went wrong with the chips and they need to find a new company to get their CPU's from, that means, unless microsoft will start mass production, either the x720 will be in stores fall 2014, or the price will skyrocket.