i have a problem when buy R710 as i need 8 GB RDIMM as 2 X 4 GB but hte technical persone refuse this and tell me you must take it as 4 X 2 GB why???????????????
First make sure if your existing memory is ECC (UDIMM) or ECC registered (RDIMM) memory. If your existing memory is ECC then that might be a possible explanation as ECC memory does not support 4GB modules (it supports only 1GB and 2GB modules). Since, you need RDIMMs I do not see a reason why you cannot install 2 x 4GB even with dual processors. Moreover, even Dell manual (refer page 129-137) has somewhat similar configuration mentioned in the Sample RDIMM population table. But in the hindsight it might also be due to the fact that the memory must be installed in pairs.
Your server supports a max memory of 288GB with Single and Dual Rank RDIMMs and the max is limited to 192GB with Quad rank memory. The max is 24GB with ECC memory. It has 18 slots in total; 9 slots per processor. The 9 slots are further divided into 3 channels (banks) - 3 slots per channel/bank.