If MS Nerd is correct, the Xbox 720 (or Xbox Next, or whatever Microsoft decides to call it) is currently being developed under the codename "Loop" -- and the machine may run on a modified version of Windows 9.
That makes it an ambitious project, seeing as Microsoft still hasn't publicly released Windows 8 and isn't expected to until sometime next year.
The new console will also reportedly be cheaper than the 360, though it's unclear if that means cheaper to manufacture or cheaper at retail. Naturally, better graphics, AI, physics, etc. are part of the package as well.
Many expect the next generation Xbox to be the first of the new consoles to debut -- unless you count the Wii U as a true next-generation machine, which few do. One hot rumor posits the company will unveil the console at E3 2013, with a retail release perhaps coming that holiday season (though early 2014 is also a possibility).
Developers (who aren't able to go on the record for obvious reasons) tell Yahoo! Games they are already at work on next-generation games for the Xbox, though they haven't provided timelines for release.
Sony's expected to release the PS4 sometime in 2014, though there's an outside chance it might target 2013, depending on conditions as well. Like Microsoft, Sony hasn't offered any details about its next-generation plans.
It's important to underscore that these are nothing but rumors at this point. If you were thinking about getting a console this holiday season, there's no reason to put off those plans. Something new is coming, yes, but no time soon.