So there's this as a caveat.
Basically, the dev kits they have now still aren't final - the documentation won't be either, the OS isn't 100% complete.
It's like everything, we won't know until the consoles are out. Even then, stuff like this might be under NDA.
Launch games will have to start going gold in as soon as 12 weeks. 12 weeks out it seems that devs would need to know how much memory was available to them.