The best way to do it is to overclock your processor as much as you can/want to and then work on your RAM timings.
Higher timings are slower, but more stable, especially for higher overclocks.
I would consider getting 2 of the exact same RAM cards though, as it tends to be more stable.
I'm not sure if lower multiplier = lower temperature, but I don't think that's the case.
What I would do is set you RAM back to default for now, OC your cpu as far as you want, and then try to tighten (lower) your RAM timings as long as you stay stable.