The RAM will be overclocked as a side effect of raising the system bus, unless you lower the divider. The fewer the modules, the fewer the points of failure. This is why you want less physical modules, so that there is less of a chance it one will not be able to stand up to the increase.
This is also the reason that dual cores of the same general family can OC higher than their quad counterparts. You can only stably overclock a CPU to the highest speed that the weakest core can sustain. Half the cores = half the possible points of failure.