DDR3 1600 MHz IS overclocked 1333MHz. Look at the stickers on your RAM modules and write down the settings shown, e.g., 1600MHZ, Latencies 9-9-9-24, Voltage 1.5V.
Then start the computer and enter the BIOS settings. Find the tab or page for the RAM settings, then set the speed, latencies and voltage to those you wrote from the modules. Then save and exit the BIOS. The system will re-POST and your RAM should be running at 1600MHz.