A user on the popular NeoGaf forums has listed the specs for the next generation Xbox Devkits. He claims to work in the Industry and according to the post, other users can confirm his occupation. Like always, the news should be taken with a grain of salt but we do know that developers have had their test kits for some time now, so who knows?
According to the user, the Xbox 720 or simply "Xbox" has an 8 core CPU, 640 GB HDD, 8 GB memory, AMD 8800 GPU and a Windows 8 Kernel OS. Remember, these are the supposed specs of the devkits. The consumer kit could have a different setup than that of the developers. The Windows 8 kernel I think is no surprise and the rather large HDD is a must too. With digital distribution on the rise, I think we'll need that much.