They increase their voltage and relax their latency to reach higher clock speeds.
Most memory controllers determine the RAM speed as a ratio of the FSB speed. You will most likely have to increase the memory frequency, memory voltage, and relax the memory timings as you OC. But that's ok, it'll be faster. Also keep in mind that a latency of 6 clock cycles at DDR2-800 is roughly the same speed as a latency of 4 at DDR2-533.