Our well-informed sources tell us that the next Geforce will not use HBM 2 memory. It is too early for that, and the HBM 2 is still expensive. This is, of course, when you ask Nvidia, as AMD is committed to make the HBM 2 GPU
....
The next Geforce - and its actual codename is still secret - will use GDDR5X memory as the best solution around. We can only speculate that the card is even Volta architecture Geforce VbG. The big chip that would replace the 1080 ti could end up with the Gx104 codename. It is still too early for the rumored GDDR6, that will arrive next year at the earliest.
http://www.fudzilla.com/news/graphics/43873-next-geforce-doesn-t-use-hbm-2----------------------------
As I said in an earlier post micron is able to push GDDR5X to 16Gbps with normal stock (not selected/cherry or special batch)
it all about noise, Power, and algorithms to handle it with the chips
so GDDR5X could be used interim until GDDR6 is avail in quantities
so that means Volta could arrive anytime this fall - preferably around the time AMD releases Vega - so they don't lose market share
however the longer AMD delays Vega the more likely Nvidia is going to wait for GDDR6 and a Jan launch