Download the motherboard manual from Asus support.
(page 2-11) Change Memory Clock Mode from [Auto] to [Manual].
(page 2-12) Set Memory Clock Speed to [800MHz].
(page 2-12) Set the first four DRAM Timing numbers.
For example, Kingston KHX1600C9D3K2/8GXhttp://www.kingston.com/datasheets/khx1600c9d3k2_8gx.pdf
Each module kit has been tested to run at DDR3-1600 at a low
latency timing of 9-9-9-27 at 1.65V. The SPDs are programmed
to JEDEC standard latency DDR3-1333 timing of 9-9-9 at 1.5V.
The DRAM Timings are 9-9-9-27 in this example.
If your memory DRAM Voltage is higher than 1.5v as in the example it is 1.65v, you need to set that option too.
(page 2-12) Memory Over Voltage [1.65]
Save and Exit BIOS