Even if you put 1600mhz RAMs, they won't run at that speed, since rams are set to run at half speed by the manufacturer. So, even if you get a 1866mhz ram, it will run by default @ 933mhz (you need to overclock them, to get the 1866 clock).But yea, you can put a 1866mhz RAM and you can acquire the 1866mhz clock(provided that you have a compatible mobo) , only by overclocking it though.
p.s: my point is that if you aren't going to overclock the RAM, don't bother getting a 1866.