I quoted 1-1.5 GB for VRAM usage considering the other 512 will be dedicated for the OS, meaning much less overhead to worry about for the games themselves. Remember, the 360 uses 512 MB of unified memory, total, that's VRAM, CPU RAM and RAM dedicated to the OS all shared.
If we're talking 1.5 GB of unified usage RAM when OS-reserved RAM is already taken care of, I can confidently assume at least 1 GB of that RAM will more often go to the GPU. That would give the Wii U the amount of memory just for its CPU that the current systems have for everything combined.
Either way it's safe to say that, if this is true, RAM will not be a problem at all for the Wii U, and if MS and Sony are preparing more modest successors as has been rumored, they might want to consider changing their game plan if Nintendo is finally getting its act together in the power department.