Those same density limitations present themselves to Sony and MS as well. While Sony was rumored to be using 2 gigs of GDDR5 that'd be 8 chips. The largest amount of chips a system has had is the 8 chips of GDDR3 the 360 launched with. Getting more than 2 gigs of GDDR5 unified (unless launching after 2013 when higher density chips are available) is going to take anywhere from 12-16 chips.
That's a pretty damn busy board.
So either Sony and MS go with DDR3 in massive quantities, or they go with a smaller amount of much much faster.
The 360 has 8 chips? somehow I thought it was only 4 there as well. So that means it's using 16bit chips as well?
How much of gain in transfer speeds can you actually get from having more chips in a real life situation?