Xbox: 2001-2005 | Xbox2/360: 2005-2010 | XB3: 2010-2015
Microsoft will begin R&D on 3rd-gen Xbox next year, if it has not already. makes perfect sense, because Sony has already publically mentioned Playstation4 several times.
Sony-Nvidia have a graphics roadmap, post-RSX and Sony-IBM-Toshiba have a roadmap for Cell that extends to 45nm and beyond with what looks like dozens of SPEs on a single chip.
XB3 and PS4 should provide near-CG (videogame CG not, big budget film CG) graphics in games. whereas Xbox360 and PS3 have turned out to be not as powerful as once thought.
meanwhile, Nintendo, after the Revolution, will most likely have a videogame machine that finally moves away from being tethered to the television.