It affects all chipsets, think about it: More memory means the CPU controller has to work harder to address it all, which as P4Clock says would absolutely have a negative impact on your CPU overclock.
For example, look at the flagship 8Pack system that costs 12k - 'only' has 16GB RAM, why? As thats the only way 8Pack can garuntee the overclock speed, if you were to specify more RAM then you would no doubt have to accept a slightly slower clock.