Well, it's not like it was exactly a secret but thanks Bethesda for confirming it. Bethesda announced it will release a new HD Texture Pack for the PC version of Fallout 4. To prepare PC gamers for this upcoming pack, Bethesda revealed the system requirements for it, and guess what they listed?

And it appears that the big publisher has leaked the name and the amount of graphics memory (8GB) that AMD's upcoming Vega10 GPU will feature. According to Bethesda, these are the official requirements for Fallout 4’s upcoming High-Resolution Texture Pack.

Recommended PC Specs

• Windows 7/8/10 (64-bit OS required)
• Intel Core i7-5820K or better
• GTX 1080 8GB/AMD Radeon RX 490 8GB
• 8GB+ Ram

Meanwhile in a new updated HBM2 specifications document released by SK Hynix they confirm HBM2 specs and availability of 4 gigabyte (32 Gbit) HBM2 stacks in Q1 2017. If you look at the photo below you can see two HMB2 stacks :) Radeon "Vega" thus indeed has 2x 4 GB HBM2 stacks, 8 GB of total memory. 

If you dig a little deeper into the document you'll notice SK Hynix H5VR32ESM4H-H1K at 1.60 Gbps (per pin), with a cumulative bandwidth of 205 GB/s per stack. x2 is 410 GB/s of memory bandwidth, if AMD sticks to the reference clock on this memory