Well here is your answer, first you are correct with the switch on the positive side, you should always switch your supply and not the ground, electronics 101 for pure safety reasons, now for current limiting, you have 9V supply but are only using 6.4, that leaves 2.6V@20mA so you take 2.6/0.02 = 130 0hm resistor but realistically any resistor from 120 to 180 ohm will be fine so it should go + into switch into resistor into lEDs into - and you are away laughing.