Recent rumours suggested Nvidia might be using GDDR5X in their upcoming Pascal series and it seems that AMD might follow suit.
AMD will probably use GDDR5X on its mid end cards while GDDR5 will probably stay with low end cards , with HBM2(the next gen of High Bandwidth Memory or in short HBM) being used in the high end and enthusiasts cards.HBM has already debuted in AMD ‘s current Fury series , featuring the AMD R9 Fury X , the AMD R9 Fury and the AMD R9 Nano.
HBM2 is expected to be available in short quantities and thats why GDDR5X will be play a big role in next gen , serving as an upgrade from current GDDR5 cards while being cheaper than HBM(2) cards.
AMD will reportedly have ‘priority access‘ to HBM2 , so GDDR5X is going to be quite vital for Nvidia.
GDDR5X is going to provide twice the data rate per memory access. Jumping from the current 32B data per memory access, to 64B data per memory access. GDDR5X is going to have a much higher I/O rates of around 10-12Gbps compared to 7 Gbps on GDDR5.