That's actually not as much as I was thinking it would have... It's only 1 more gb than what the x1 currently has. Luckly the memory bandwidth has a good increase.
XB1 only has 5GB usable by developers, the other 3 GB is reserved for the OS/UI. This has 9 GB usable by developers, an increase of 4 GB over XB1/PS4, and an increase of 3.5 GB over PS4 Pro.