OWC sent me a 32GB kit (8 X 4GB) to try in the new Mac Pro Nehalem.
I don’t really need this much memory, but anyone working with huge files in Photoshop should use 32GB. A good compromise if you don’t need the full 32GB is “only” 24GB: lower cost and (in theory) slightly higher performance.
But when eight 4GB modules are used, memcpy performance is about 15% faster than with eight 2GB modules, and in fact 75% as fast as the six module configuration. In other words, an eight module configuration of the 4GB modules offer 75% of the maximum performance, a bonus upside, even if real application are unlikely to show no difference.
One caveat: the OWC 4GB modules will not work with other modules of any brand or capacity This is unfortunate, but it’s not clear that Apple OEM modules might not have the same limitation. And at 1/3 the price of Apple ($6100 for 32GB), it’s a quibble.
The good news is that anyone with the new single-CPU (4 core) Mac Pro Nehalem can use 4GB modules to get to 16GB, a critical consideration for some users.