Witness some of the magic of DDR-II:
"The first and foremost important aspect is that a slow core can be used to achieve high data frequencies. For example, a 100 MHz DRAM core can be used to achieve DDR400, a 133 MHz DRAM core can output data at 533 MHz. Related and second in line is the fact that a lower core speed will result in lower power consumption since power consumption is a linear factor of the operating frequency. Third, because of the lower frequency, it is possible to lower the operating voltage from 2.5V to 1.8V."
Hardware geeks head right here.