I have an DP55SB mobo, I have 12 GB CORSAIR DDR3 XMS 3 1333 MHz 1.5v sticks installed, and she boots up fine. She is capable of handling 16GB's of ram. But when I install the other 4GB stick, I get a 3 beep memory error. Can anyone tell how to install the other 4 Gb's.Any help would be highly appreciated,Thanks.
The problem that you are having could be related to a lack of power from the power supply. Try using the system with a larger power supply, to discard that as the cause of the problem.
Also, test your system outside of the chassis. Please place your motherboard on a table, on a non-conductive material, and try booting it with only basic hardware attached. This is to ensure there are no grounding issues with the system case.