The latest model of the Mac Pro has 4 slots for memory. Each slot can hold a 32GB module for a maximum of 128GB. Important note you can not mix the 32GB modules with the original system memory.
The DDR3-1333 DIMMs for the Mac Pro require a Thermal Sensor for full performance and compatibility. Many 3rd party Mac Pro DIMMs do not include this sensor. Eagle memory always conforms 100% to the Apple spec, and these DIMMs include the Apple Thermal Sensor.
Tri-channel memory configuration gives a slight performance advantage. The DDR3-1333 DIMMs for the Mac Pro will run 1-3% 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.
Non-Mixable 16GB and 32GB DIMMs: 16GB and 32GB DIMMs can ONLY be used with identical 16GB or 32GB DIMMs from the same batch (i.e. ordered at the same time.) 16GB and 32GB DIMMs can NOT be mixed with each other.
Matched Modules: The Eagle Memory kits for the Mac Pro DDR3-1333 contain a matched set of DIMMs. The kit will consist of identical brand/model/spec IC's. They will be from a single batch, and will have matching series numbers. This ensures maximum performance, and perfect compatibility.