NVIDIA skips first-generation HBM and jumps straight to HBM2, which offers a memory bandwidth of 1.2TB/s, over 3.5x as much as the 336GB/s GDDR5 memory found on the GeForce GTX 980 Ti. TweakTown claims Pascal is going to be the most exciting release in NVIDIA's history with great improvements in terms of performance and power efficiency.
We should also expect to see between 16GB and 32GB of HBM2-based VRAM to be offered on the new Pascal-based cards, where I'm sure NVIDIA will up the Titan X 2 (that's what I'm calling it for now) to 32GB from the 12GB of VRAM found on Titan X. I think we'll see the GP104 (the smaller chip) arrive as the GeForce GTX 1080, but I'm going to throw this rumor out there: the numbering system will change for this generation. I've just explained above how this is the most important release from NVIDIA ever, with 16nm, HBM2, new architecture and more - so start getting excited, folks!