CAS latency is the number of clock cycles it takes a stick of RAM to respond to a call; this is taken into context together with memory speed. All things being equal, the lower the CAS latency, the better. Your first consideration, however should be memory speed, then look for the ram with the lowest cas. The lower the cas, the more expensive it will be. If you start tightening your timings, you're gonna need to incrementally increase voltage to the ram. Refer to the maker's voltage recommendations so you don't fry your ram. The cas/speed is multiplied by 2,000 to find latency in nanoseconds. Which means that slower ram can have lower latency if it has lower cas latency than faster ram with higher cas latency. Again, cas latency is the number of clock cycles it takes to respond which is affected by the speed of the ram. Also, your cpu & mainboard will determine what's the fastest ram you can use, then it's a matter of how much you want to shell out for the lowest cas latency for that given ram speed.