RAM speed usually isn't worth more than 2-3% worth of performance, additionally some games respond better to faster speed RAM and some respond better to tighter timings. RAM quantity is very nice in many more cases than RAM speed is as speed can never make up for quantity. That said, if you used both it would run all the RAM at the lower speed. So, having 8 GB isn't bad, you'll be fine in almost all games.
Another consideration is motherboard and CPU support. If your system doesn't support overclocking the 1866 RAM is going to run at 1600 anyways. The tighter timings won't give you a whole lot of gain, again we are talking low single digit percentages. It might be better to just clock it all at 1600 MHz CL11 and use it all.