I hear that 1866MHz DDR3 memory for the new Mac Pro is likely to cost no more than 5-10% more than existing 1600 MHz memory, which is little different in cost from 1333 MHz memory.
So that means filling four slots in the new Mac Pro with 64GB might cost ~$825 (based on about $755 for 64GB of 1333 MHz).
That’s good news given historical changes in which the latest memory was hyper expensive.
Update: OS X Mavericks due out this fall is stated to support 128GB memory.
And OWC tells me that 32GB modules can be built at considerably higher cost, so it seems that 128GB will be the real limit, assuming funds are not the limit. Also:
All those that have 48GB / 96GB from us now... they will be able to add and use 16GB / 32GB more with the existing Mac Pros under Mavericks!
In short, assuming Mavericks fixes the OS X memory addressing bugs in Mountain Lion, a 4/6-core Mac Pro will be able to go to 64GB instead of 48GB as today, and a 12-core from 96GB to 128GB.