Another consideration that isn't often reported/discussed is Intel's selection process/QA for processors. In other words, what makes it a Xeon?
Core i7's are mainstream chips (btw, quad core isn't old technology, it's very current and is in use in the majority of i7's). When the chips are fabricated, they're run through a battery of tests for things like fault tolerances, speed rating, etc etc. Chips that pass all tests at the highest speed rating are sold as higher speed, ones that don't pass are re-tested at lower clock speeds, etc and the ones that don't pass at any shipping speed are scrapped.
Within each speed class, it's not simply a yes/no or pass/fail test. The processors are scored and ranked. The top of the heap (top 1-2% of the class, as it were) are pulled aside for additional testing and review. They're 'hand-binned' (meaning a human gets involved and additional tests are run to insure they'll perform well under heavy workloads and extended periods of time. The cream of that crop is what ends up getting boxed and sold under the Xeon brand, and it's that additional level of QA scrutiny and testing that helps explain/justify the increased cost.
On top of that, of course, the dual processor Xeons have an additional high speed QPI pathway, which is essential for them to function in a dual processor configuration.
Speaking to the issue of memory, just because Apple doesn't offer it on the config screen doesn't mean it can't be done. The 2009 machines can definitely accept 8GB DIMM's (for a total of 32GB on the quad-core, 64GB on the octo-core), the 2010 machines should be able to do the same.
Even more interesting, earlier this year Samsung began sampling a 64GB DIMM. It's not expected to ship in any real quantity until either late this year or early next year, but yes, that's right, 64GB on a single DIMM that's capable of fitting in a Mac Pro memory slot. Expect it to be insanely expensive for the next few years, but ultimately the production of that capacity of RAM module will have a trickle-down effect on memory pricing, and in a while we can look forward to dirt cheap 4GB RAM and almost affordable 8GB modules.
also, 16GB dimms will work in the quad and dual.. giving max memory of 64GB for the quad and 128GB for the dual.. I think westmere supports a max of 128.
