Thanks for your question!. please follow the below rule of thumb for NHM EP based system
Follow these rules for peak performance:
•Always populate both processors with equal amounts of memory to ensure a balanced NUMA system.
•Always populate all 3 memory channels on each processor with equal memory capacity.
•Ensure an even number of ranks are populated per channel.
•Use dual-rank DIMMs whenever appropriate.
•For optimal 1333MHz performance, populate 6 dual-rank DIMMs (3 per processor).
•For optimal 1066MHz performance, populate 12 dual-rank DIMMs (6 per processor).
•For optimal 800MHz performance with high DIMM counts:
–On 12 DIMM platforms, populate 12 dual-rank or quad-rank DIMMs (6 ) per processor.
–On 16 DIMM platforms:
•Populate 12 dual-rank or quad-rank DIMMs (6 per processor).
•Populate 14 dual-rank DIMMs of one size and 2 dual-rank DIMMs of double the size as described in the interleaving section.
•With the above rules, it is not possible to have a performance-optimized system with 4GB, 8GB, 16GB, or 128GB. With 3 memory channels and interleaving rules, customers need to configure systems with 6GB, 12GB, 18GB, 24GB, 48GB, 72GB, 96GB, etc., for optimized performance.
answer to your questin, yes you can mix DIMMS(differ size) provided all DIMMS are identical. let me know if you have any question..
First you may want to check with Dell about supported memory configuration. On the CPU and chipset side, you can try the
Intel Memory Configuration Tool for optimized memory configuration.
Detailed memory population requirements can be found in Intel® Xeon® Processor 5500 Series Datasheet Volume 2