Microsoft said something a while back about the nextbox that it will be Forward Compatible. Meaning you will be able to play a new game from the nextbox and still play it on the 360 but, it will be scaled down to the 360's specs .
That way Microsoft can still keep the current usebase happy by playing a Gears 3 on the 360 thats design for the 720's capabilities.
also it gives the Xbox userbase a choise to those that will like to upgrade and those that not and still play the same game on either consoles on Xbox Live or SP mode.
If you play a nextbox game you will get the full experience, better graphics and so on.


My Trigger Happy Sixaxis controller








