If you mix memory modules with different clock speeds, the mobo will automatically throttle the faster clock down to the slower clock. So it is pointless to buy 2133 MHz RAM if you intend to use it at the same time as the 1600 MHz RAM.
Generally speaking however, for gaming you don't really need anything higher than 1600 MHz. You might see a slight improvement going to 1866, but definitely after that the return on investment is vanishingly small.
You don't need to overclock your CPU if you memory is overclocked.