Is DDR3 memory backward compatible with DDR2 and DDR?
DDR, DDR2 and DDR3 memory modules are mutually incompatible. Voltages, pins and signaling differ between the memory types. Note the “Key” hole on these memory modules are positioned differently, as shown in the figure below. If you cannot get memory inserted, you should turn it 180 degrees and try again or check to make sure you have the correct memory type for your desktop board.