You should be able to just set the Memory Clock to X6.66 to get your 1333MHz. You might have to up the voltage to get stability and when you unlock the voltage control you will get a warning about overvolting. You don't need to worry too much about this as it is standard and as your RAM is rated at 1.8 you will be OK. You might find that you can get away with a lower voltage setting for your RAM but it will be a case of trial and error to find the right amount.
