I wonder if AMD will ever move onto an analogue of their CPU modules with their GPUs, and come up with a dual GPU design that shares the memory. As I understand it in SLI/xfire a lot of the same stuff is mirrored on the memory of each card to prevent tearing; well imagine if you had 10 gb of shared memory that you could actually use all of it. I imagine that would require a few major changes to the pcb and GPU dies to allow direct communication between them (which if I recall, the 4870x2 was supposed to have some sort of pipeline that was never utilized).

I have no idea if this is feasible or even worth the R&D costs. However, I think 10gb of shared memory would be really useful for a multi 4k monitor system (and I'm guessing if you can afford a gtx 790, you can afford the monitors).