From what I saw with Windows 2012 so far, it doesn't require much more memory than Windows 2008, so I'd probably start with the memory sizes I mentioned before and only increase is if required. For database servers it's always a question of what's running on them. They usually grab most of the available memory (unless you restrict them) to optimize data access. With the physical memory you have and the current usage, you may start with e.g. 8GB for the database servers and check how they behave.
André