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The updated 12-core Mac Pro system is built around two 2.4GHz 6-core Intel Xeon E5645 processors. These processors are based on 32-nanometer Westmere architecture and were first produced during the first half of 2010. They're good processors, but I think we’d do better to go with a processor from the newer Intel "Sandy Bridge" line.
I've chosen the Intel Xeon E5-2430 6-core processor. This is a 2.2GHz part that can be turbo boosted to 2.7GHz and makes use of the LGA1356 socket. It's a bit more expensive than the E5645 but it's also better.
Remember that you need two of these processors.
Price: 2 x $560.