According to Intel, the 3500 series processors can handle 4GB memory sticks. Whether the new 4-core Mac Pros can handle them is uncertain. Did you notice that for the 8-core machines, Apple sells the 4GB sticks for $762.50 each, plus the trade-in on the six 1GB sticks? Maybe that's why they don't offer them on the low-end machines. Nobody would buy them at the price they're asking!