So for example out of these two;
i5 - 3570 with 8x8GB DDR3 1600Mhz ($375)
i5 - 3570k with 8x8GB DDR3 2400Mhz ($450)
The K series would allow the CPU's memory controller to utilize higher Mhz RAM, therefore the 2400Mhz RAM wouldn't be bottlenecked, but the extra $75 isn't worth the performance...