depends on what your comfortable with. i think most try to stay below 1.60.also if you keep things cool (good airflow) you should be ok w/1.69 i think. im probally not the best person to be asking cause i dont like to go to high on any of the voltages.right now im using pll1.57 and im trying to lower it to 1.5(not comfortable with 1.57)and vtt not really sure but i keep mine below 1.4.but still not 100% sure . please feel free to correct me if wrong.