Phykell, The answer would be based on the amount of processors installed. In populating the server the guidelines you need to adhere to, and make sure the new dimms aren't violating is; 1) you can't mix Rdimms and Udimms, so ensure that they are the same. 2) is that when populating you must make sure the channels match. 3) The larger dimms must be 1st in the channels. So hypothetically if you were to have 2 processors, 24 dimms (8 16GB and 16 8GB). You can place the 16GB dimms in the slots with the white tabs and then the 8GB in the green and black lots. Giving you 256GB, I am not certain the amount of memory you wish to go to, so I am just giving examples. You could also do a smaller configuration of 8 16GB in the white tabs and 8 8GB dimms in the black slots.  You can find additional guidelines and suggestions on page 40-46 of this manual - [URL] Let me know what your configuration is and what you are seeing. 
Hi, I've got a dual processor R620 server with 8x 8 GB RAM modules giving a total of 64 GB. I now want to upgrade the memory and have seen that the best module to buy (GB/cost) is the 16 GB part. Can someone tell me what options I have? Can I keep the existing 8 GB modules in the server and add in additional 16 GB modules? If so, what combinations can I add?  For example, can I only add 4x 16GB, 8x 16GB 12x 16GB or can I get away with other combinations? Thanks for looking :) 
RAM Upgrade Recommendations for R620