I think it would be unlikely and silly to outright require 1866Mhz RAM.
So just 1600 MHz will be fine, because it says on game debate (http://www.game-debate.com/hardware/index.php?gid=489&graphics=Radeon%20HD%206530D) that it only supports 1866 MHz RAM
You could always get 1866 RAM to be sure. You don't need to actually
SET the RAM to run at 1866. 1866 is just the frequency the RAM has been
tested at and proved to work at fully. Thus you could experiment with the
likes of 1600 too.