Increasing the BCLK also increases the RAM speed, and IIRC, the QPI too. You can increase BCLK and set the memory multiplier to a lower ratio, and that will keep the RAM speeds down. For example, the stock memory multiplier on most X58 boards is 8x. Thus, if you lower it to the lowest possible, 6x (it actually says 2:6, but means the same thing) on my EVGA X58 SLI LE for example, differs on other boards, then you can run the BCLK at say 200MHz, resulting in an approximate ~ 4GHz overclock without having to overclock the RAM, if you use PC3-10666/1333MHz modules, as the RAM will be running at 200MHz x 6 which equals to 1200MHz.
RAM can still stop overclocks on P55 boards where the memory multiplier is quite limited I think, I could be wrong though, and you're stuck with a 8x multiplier, which means to get 4GHz with a 20x CPU multiplier you'll need to have RAM which can run at 1600MHz. With bigger overclocks, you'd probably be more interested in RAM speed, but latency imo is more important.