Best performance requires six or twelve DIMMs of identical size. Lots of combinations are possible, but most would be sub-optimal, some drastically so.Does anyone know if 64GB of RAM is a configuration possibility?
64GB is not listed on Apple's Tech Spec page? https://www.apple.com/mac-pro/specs/
Is each of the 12 user-accessible Mac Pro DIMM slots its own memory bank?
Four or eight DIMMs, properly arranged across four of the six available channels would only give about two thirds of the max memory bandwidth, for instance. The 4x8GB config of the entry level model falls into this category.
So Apple will only allow a small subset of possible configs when upgrading, those with the highest performance, 6/12 identical DIMMs. Hopefully Apple will have a technical document available describing the relative performance of various configs, for those who upgrade on their own.