With 1866 and below DRAM best to to sticks with JEDEC standard 1.5 sticks, 2133 and up 1.6-1.65 is OK as you will prob have an OC and the MC (memory voltage will be higher) I try to keep them within about 0.60 of each other....also 1.65 for 1600 is often indicative of weaker memory ICs (i.e. WHY would they need 1.65 to run at 1600? when the far majority of even 1866 run at 1.5