Now that you mention Natal, I have a conspiracy theory.
When Natal appears, I am expecting Microsoft to do something similar to what Nintendo has done with the DSs and Sony has done with the PSP - hell, Nintendo has done it with the Wii, in that, I expect a totally new XBox (at least from the design perspective) model with a new name (it will not be called 360). It will also have Natal built in. How far they then go to expanding its capabilities I could not guess, but it would not surprise me if they added extra RAM, GPU bandwidth, etc. The same game will run on both the new machine and the old machine, but you will be able to expect higher performance and possibly extra features on the XBOX N3.
They are due a serious hardware revision in any case, as in the PS3 Slim, and I think that it will all be done at the time Natal is launched.