I have a motherboard that supports DDR3-1333/1600/1866 RAM. Could I put two DDR3-2133 sticks in the machine and just have it perform at a lower speed, or would this not work? The RAM currently in the machine is two 4 GB DDR3-1600 sticks, and I was looking to buy more to add on to this. Should I buy two more 4 GB sticks that are the same speed (1600), or would it be okay to exceed this and have the other sticks perform at 1866? Sorry if that's complex, but I hope you understand what I mean. In the end, I'm looking to upgrade to 16 GB of RAM total. Thanks!