DDR and DDR2 memories are classified according to the maximum speed at which they can work. But, besides the speed, there is another information that tells you the memory performance: timings. Timings are numbers like 2-3-2-6-T1, 3-4-4-8 or 2-2-2-5, the lower the better. In this tutorial we will explain you exactly what exactly each one of these numbers mean.
This is a great article if you want to better understand ram timings better. It may even help you understand why your memory overclock can sometimes go horribly wrong.