I have a ASUS motherboard set up with 12 gig of 1333. can I populate the other three with 1666? I am more concerned with stability than speed. Am I right in thinking manually setting all tyeh memory to the 1333 specs is best practice?
More aboutmixing memory speeds ddr3 1333 1666
Yes you'll need to run your ram on the slower of sets rated speed and timings/voltage.
Whatever you've got 'NOW' get more the 'SAME EXACT part numbered RAM.' Do NOT mix-match your RAM! Especially never mix: Speed, CAS timings, and Voltage. The typical difference between 1333 and 1600 MHz RAM is 1~3%