DDRx Speed

Clock Rate (MHz) is the reciprocal of Clock Cycle (nSec)

and 

Clock Cycle (nSec) is the reciprocal of Clock Rate (MHz)

DDR is defined as double data rate, which means the data bits are clocked on both the rising and falling edges of the clock. 

DDR3 with Clock Rate of 800MHz would have a Data Rate of 1600.

Example of manufacturer specs.