According to IGN, the next version of the XBox will arrive on store shelves in late October or early November of 2013. The GPU will be based off of the AMD 6000 series. Given that, the graphics processing will be six times more powerful than the curent generation and 20 percent more powerful than the forthcoming Wii U. The article also mentions that the GPU may be based of the Radeon 6670, which is capable of multidisplay support, 3D, and 1080p output.
Dev kits should reach developers sometime in August.