Update August 12: OWC Larry says “85/15 chance on 8GB working in 4/6-core— I share a cautious optimism now with comparison/expectation to the 2009 model design chipset and expectation from”.
Update August 11: several sources suggest (in various ways) that 8GB modules should work in the new 4/6-core Mac Pro because it should be using the Intel 5520 chipset, a minor update to the Intel 5500 chipset in the 2009 Mac Pro. I can’t verify this, but it does sound promising. Another source says certain vendors are already claiming compatibility (not a great plan to claim before actually testing in production machines, as Apple has a history of making slight changes to memory requirements).
Your smart move is to wait 10 days or so to see if 8GB modules will or will not work in the new 4/6-core Mac Pro. Or whether Intel’s stated maximum of 24GB is simply a specification, but that 32GB will work as with the 2009 Mac Pro, in spite of the specification.
Some readers seem to think that 8GB modules will work, and that some vendors are already claiming they will. I’d be cautious until the new Mac Pro ships, and those vendors actually test their modules in them.
There does seem to be some room for play here— while Intel specs for the hexacore W3680 state 24GB as the max memory size, the addressing is 36-bit, which should allow 64GB (32GB if one bit is reserved). There is also the vague “dependent on memory type” parenthetical.
The previous Mac Pro 2009 lineup allowed the use of 8GB modules for 32GB total (though Apple never offered more than 16GB using 4GB modules). I personally use 24GB as 3 X 8GB in my quad-core 3.33GHz 2009 Mac Pro, but I’ve also run it with 32GB (ditto for the 2.66GHz model I had for some time).
Apple has made the decision in the 2010 Mac Pro to (a) use a design with four memory slots, and (b) to use processors that are spec'd to max-out at 24GB with six slots. Let’s hope that specification is simply Intel being conservative, and that 32GB are possible.
Presumably this fattens profit margins in two ways: lower priced chips for the 4/6-core, and pushing anyone needing more memory into an 8-core or 12-core model. But it disrupts loyal users who just need more memory, not 8 slow cores or 12 moderate speed ones. Here’s another take: Apple Loved You Pro Users; Loves Your Money More, Now.
It remains to be seen if 8GB modules magically work in 3 slots to provide 24GB, but it’s clear that 32GB will never be possible in a 4/6-core 2010 Mac Pro, at least according to Intel chip specifications.
Rob-art over at barefeats.com pointed out this Intel specification table for the processors being used in the new 2010 Mac Pro. Intel shows 24GB as the limit for the 4-core and 6-core processors in the new Mac Pro. But that assumes 6 memory slots, and those models of the Mac Pro have only 4 slots, hence 4 X 4GB. The outside chance of 24GB would thus be possible only if 8GB modules were to function as 3 X 8GB, a possibility I’m told is deemed unlikely.
Another interesting difference in the 2.8 and 3.2GHz processors is the lack of “Intel® Trusted Execution Technology”, a security feature. Whether Mac OS X uses it, I don’t know.