In each generation of hardware, the rules change. I was under the impression that the last generation of intel processors were using two banks of memory, and that you would get significantly better performance therefore if you had both memory slots populated.
To be specific, in a Dell Vostro 3750 for example, there are two memory slots. Dell sells this laptop with 4Gb in one slot, or 6Gb with two slots, or 8Gb.
Is there significantly less memory bandwidth in the configuration with only one memory module? And in those with 6Gb, do the accesses beyond the 2Gb of one module happen slower?