I dont think it will happen that soon, maybe 2011-2012. MS has invested so much in its current system, and introducing a new XBox this early in the Gen would severely hurt its creditability. Currently I love my X360, and would be a tad upset if a new system was introduced next year. But hey, that's me.
Secondly, the current cards out do not show a significant (I say significant) upgrade to the cards currently in the X360/PS3. A bit more development time and we could see a much better graphical improvement.







