Sadly the xbox skimped on memory, both main and video, due to costs at the time. We are saddled with all those legacy constraints for the next couple of years.
From what I remember they were going to go 128/128 on the 360 until the PS3 umped the Ante. We should all be very happy that MS decided to "splurge" on increased memory.
Don't forget the 20MB of edram (I believe) that is onboard the GPU. As the next gen consoles are likely looking at native 1080p as an actual target I don't know how feasible it is going to be to have that cache on board.
That said, it will be interesting to find out what they do with the xbox next. Are they going to keep Xenos around for legacy apps and a low powered dashboard/video playback GPU? Can it gain some compute ability that would allow it to be used for physics/AI in addition to the main GPU? Will it be able to do some sort of overlay or hybrid crossfire?
Or will they just remove it and tell you to buy a used xbox if you want to play last gen games at all?
I am hoping they don't just die-shrink Xenos, physically double or quadruple its architecture and push it relatively cheaply out the door. A full DX11.1 part would push both the console and the PC gaming markets in a way that would be more beneficial, IMHO.
If it is GCN based, that would be pretty darn awesome and much needed (again, IMHO) in AMD's cap. We would likely see the same "OMG use the xbox as a supercomputer!" as the PS3 experienced early in its life.