Double it, since there's 16GB DIMM's out. So up to 64GB for the SP systems, and 128GB for the DP units at launch, until 32GB sticks can be had (they were announced some time ago).
Couldn't you just add those 16GB DIMMs to the old Mac Pro too? I guess it might be off-spec, but once Apple Care is gone, that doesn't matter much.
Would be nice to see more slots, but I guess that gets into too many technical details to change easily.