Likely, if you add two 1600 DIMMs, they will operate at 1333 because of the speed of the other DIMMs present. Currently (I will presume), you have your two DIMMs in separate channels. When you add the new DIMMs, you want to move the two existing DIMMs to one channel and put the new DIMMs in the other channel.
As always, my recommendation is to find a way to test with extra DIMMs installed before you actually purchase any...
Hope this helps,
This message was posted on behalf of Intel Corporation
Please bear in mind that the memory controller hub is part of the processor now, so, memory speed will also depend on the processor model.
You can check supported memory speed at ark.intel.com, you need to select your processor model.