According to lherre, the are some Xbox dev kits that already have a 7000 series Radeon. And the final hardware could end up with a 8000 series Radeon. The russian "insider" seem to suggest the latest dev kits have TWO Radeon 8000. If the 360 dev kits are any indication, as they also had teo GPUs, this could mean the Xbox 3 GPU is omething hybrid more advanced than the GPUs which are currently available.
We don't know what level 7000 there is, it can be the same or even faster than the PS4 7000 series GPU. We just don't know. Also, if this "russian insider" rumour is true, that would make the 720 quite a lot beefier than PS4.