Your old '665' DDR3 is prob it's true freq which means it's actually a data rate of 1333 since this is DDR (DOUBLE data rate) as DDR3 starts at a data rate of 800 (true freq 400). As far as mixing new with old it's a 50/50 crapshoot, there are no guarantees when mixing sticks, even if you get one identical to the old.