It's best to have a 2x4 stick at 1866mhz cl9, that's assuming you are building a gaming pc .Because by having 2 sticks, the board can utilise dual channel, so instead of drawing the processing power from just one slot, the 2 separate sticks allow it to draw the power from both, `866mhz is better imo, there are tests that show that any speeds above 1333mhz don't make a difference, but 1866mhz at a latency of 9 is considered by many as the sweet spot for gaming,