exzited, you tend to get the most noticeable gains with CPU-NB overclocks after you've set a CPU overclock. Having said that, it might be a good idea to explore just how high you can increase your CPU-NB frequency. Some chips allow for just simple CPU-NB multiplier increases; others require a combination of multiplier + frequency changes (this is the far more common scenario).
If you're content with keeping your 4.2GHz CPU frequency, then do it via the method that would require the least amount of voltage to keep stability (in your case, a multiplier increase works best, as you stated). But if you want to get more quantifiable (both in benchmarks and at the "keyboard level" - differences you can perceive) increases in performance, I would explore just how much you can increase your CPU-NB speed.