That's exactly what 360 did...it can be done.
Actually 360 /Xenos pushed so close to the bleeding edge it was kind of ridiculous, Xenos used some tech that wasn't in ATI cards for another couple of years. It also wasn't finished until the very last second which is an incredibly dangerous game to play. Xenos was basically ahead of it's time, forget contemporary with it's time. The equivalent of a 2014 video card in a 2013 machine.
The problem is if you try to time it just right like MS did with Xenos, if there's a problem you literally have no choice but to delay your console for months.
Basically it can and has been done, but I wouldn't recommend it and dont really see it happening this time. I think they'll go with something either even with the time they're aiming for (late 2013 card for late 2013 console), or slightly behind (early 2013 card for late 2013 console, etc).
But a 2012 card in a 2013 machine, I dont see.
It can be done yes, but this is also assuming Microsoft follows the same business model that lost them
a very large amount of money for 2 generations.
I foresee them enjoying making money more so than losing money, and their recent business model of Kinect shows which direction they're looking to.
The only main reason the 360 has had a resurgence in popularity was because of Kinect.
I think you're right to assume the next Xbox will be a more capable machine than the 360, as Microsoft understands the "core" gamers are the backbone of their (and arguably any) video game business.
But if you're expecting a very large jump, you are going to be very disappointed. I don't know if this has been pointed out to you before or not, but you should temper your expectations of the next Xbox.
Keep in mind, as zombie pointed out, in a closed system environment a system can be capable of much more; case in point the 3DS hardware can pump something out like this:
(Resident Evil: Mercenaries 3D)
compared with this:
(Resident Evil: Mercenaries Vs, optimized for iOS 3GS, *4 and 4S)
*4 was the primary platform target due to Retina compatibility although it was designed with 3GS in mind. Runs better on 4/4S. And, this game was actually
based off of the 3DS version.
We all know how on paper an iPhone 4, even a 3GS seems much superior tech-wise to a 3DS, but the "closed" nature of the system is very important. (Never mind Revelations vs Degeneration iOS)
But this is now getting tangental. My point is the closed system environment can play a huge role.
But, that said, still do temper your hardware expectations. The rumoured "6670" in the "720" devkits makes a lot of sense to me. Its a much more capable card than Xenos. But don't expect a super powerful chip.
Also, while I am no tech head in any way, I do agree with wsippel's stance regarding the DSP. It is a smart engineering design move.