limits to memory on amd64

During the discussion at MeetBSD the question came up as to what the real
limiting factors were with regard to how much RAM a system could have.
it was put to us that the limit was currently around 512 GB, though no-one
at teh discussion knew what the mechanism of the limitation was or 
what might ligh
beyond it.

Could anyone who knows, pipe upt and let use know what the factors 
are, and if the
current limit is overcome, what the next one after that will be?

Julian