The reason it is doing that is because the default for the PhenomII is 1333MHz, you will have to overclock the memory to get it to 1600MHz.
The first step will be to go into the BIOS (Delete key immediately after powerup)
Go to "Advanced Mode" in the top right corner.
Go the the "AI Tweaker" tab
The 7th box down will be "Memory Frequency"
Change that from "Auto" to "1600"
F10 to save, and restart