Memory Overclocking

Hi

I recently bought the asus M4A785TD-V EVO motherboard : http://www.asus.com/product.aspx?P [...] templete=2

The specs indicate that it supports DDR 3 1800 mhz (O.C) memory.

I have a pair of corsair dominator ddr 3 1800 memory modules.

The problem is that i cannot get it to overclock the memory to 1800 Mhz.

The settings in the bios is 400 , 533 ,667 ,800.

My current setting is 800 . In the startup screen it shows that the memory clock at 1600 mhz.

How can i get it to clock at 1800 Mhz ??

Any help will be appreciated