When one talks about memory timings they're basically talking about how long the system has to wait for the memory to be in a ready state before data is fetched or delivered. With all things equal, a stick of DDR memory capable of running 2-2-2-5 will make the computer operating experience seem faster than a DIMM which may only run at 3-4-4-8. This is because the delay from when the memory receives an instruction, retrieves the data, and sends it back out is less. Where it starts to get confusing is when you has the choice of buying high speed memory with slow timings...