Welp, you're not gonna get that with GDDR5 anytime soon.
A 512-bit DDR3 bus would be needed to get anywhere near the same bandwidth league (still shit). DDR4 is going to be pretty low bandwidth initially as well, but at least you'll have stacking options to more easily hit 8GB.
What would be the size limit for a console memory bus? (Based on Samsung's GDDR website) if you can tolerate a 256-bit bus with 8 GDDR3 chips, you could get 4 GB of memory (assuming 4Gb GDDR3 chips are available in 2013) with a bandwidth of 83.2 GB/s (once again using Samsung's 2.6 Gbits/s quoted speeds)
If you look at their GDDR5 numbers , you could put together a 4GB system with a 256-bit bus and ~224 GB/s bandwith, but requiring 16 chips (and assuming only 2 Gb densities will be available next year).
Dropping the bus to a 128-bit limit and only 8 GDDR5 chips, you're limited to 2GB and about 112 GB/sec.
So then what's preferable:
System 1:
4GB GDDR3 with ~80 GB/sec bandwidth
System 2:
2GB GDDR5 with ~100+ GB/sec bandwidth
If we're looking at a reasonable console GPU and I consider the HD7770 one, it only has (needs?) 72 GB/s of bandwidth to get the great performance that it does. I'd take the 4GB of GDDR3.