Hi, the best way is for all of the memory to be exactly the same.
I just aquired a Dell R510 with dual X5670 I also got 8 stick of RAM(4x) DDR3 RDIMM 4GB 1RX4 PN: M393B5270CH0-YH9(4X) DDR3 RDIMM 4GB 2RX4 PN: M393B5170FH0-YH9 As far as i know they have the same spec expect the number of memory rankThe R510 seem to have a weird memory config with 2 banks separated in 3 channels. BANK1 (slot 1 2 3) BANK2 (slot 4)Channel 0 (slot1) (slot4)channel 1 (slot2)channel 2 (slot3) Im planning to run this in Optimizer mode.Any idea how i can populate all the slot with my current memory without gettinga geometry of a unsupported dimm error? Or i just need to get 4 more stick of one or the other mem config? thanks in advance!  
R510 memory issue