The 1.5 is aimed at a stock CPU running 1600 sticks, I generally recommend no more than 1.5 for 1866 and under sticks, but 1.6-1.65 is perfectly fine for 2133 and higher sticks - which sticks are you looking at (the Trident's I hope, if so great sticks, I've got the 32GB set of 2666/11)