This RAM is correct for use in the brand new Westmere 12-core 2.66Ghz and 2.93Ghz, and the 6-core Nehelam 3.33Ghz. This DDR3-1333Mhz Mac Pro memory is compatible with the 4rd Generation model. The DDR3-1333 DIMMs for the Mac Pro require a Thermal Sensor for full performance and compatibility. Many 3rd party Mac Pro DIMMs not include this sensor. Gigaram memory always conforms 100% to the Apple spec, and these DIMMs include the Apple thermal sensor. The DDR3-1333 DIMMs for the Mac Pro will run 5-10% faster if they are installed in matched sets of three (tri-channel). You may add DIMMs in any combination, but performance will suffer slightly if the DIMMs are not matched 3x at a time.