Not much is known about Microsoft's next Xbox, codenamed Durango. It's been long-rumored to use AMD chips for both its CPU and GPU--a point that's been repeated by financial news service Bloomberg. Speaking to an anonymous source, Bloomberg reports that the next Xbox will use a variant of AMD Jaguar technology, and that the switch to x86 architecture (much like PS4) will mean "game discs made for the current Xbox 360 won't be compatible."
Sony's switch to x86 means that PS4 won't be able to play PS3 games either, although it plans on using Gaikai tech in some way to help in the transition.
Although Microsoft has yet to officially talk about the next Xbox--except to apologize over comments about the system being "always online"--The Verge claims that Microsoft is planning a reveal event for May 21st. What The Tech also reiterated that date, whilst also throwing some more tidbits into the rumor mill. BlockheadBrown has a quick summary in Chatty.