Quad-Core Nehalem Mac Pros Support 16GB RAM
According to Apple's official specs, the Quad-Core Mac Pros only support up to 2GB modules in its 4 memory slots resulting in a maximum installed memory of only 8GB. This finding by OWC means the maximum installed memory in these new machines is at least 16GB. While the cost of 4GB modules is somewhat prohibitive at the moment, this should ensure future expandability as the price of memory drops.
In contrast, Apple's 8-Core models carry 8 memory slots total and can officially support 4GB modules bringing their maximum installed memory capacity to 32GB. This advertised 8GB max vs 32GB max RAM discrepancy between the Quad and 8-Core machines has been a major issue for some customers. Apple has been known to under-advertise the maximum ram capacities for their machines in the past.