Hi I'm going to build my first pc and will be using 16gb of ddr3 ram. The thing is that I see different mhz like 1600,2100 and 2400. Can someone please tell me the differance between them and if I could also mix them together. In going to use 2 4gb sticks of 1600mhz of ddr3. And also 2 4gb sticks of 2400mhz of ddr3. Thank You.