CAS Latency is the time it takes for the memory to output data after receiving a read command and it is measured in clock cycles. CAS2 is latency of two clock cycles and CAS3 is latency of three clock cycles. CAS2 is one clock cycle faster than CAS3 and result is better memory bandwidth, Ex. 100MHz FSB@CAS2 is about the same performance of running the FSB at 115MHz@CAS3. One of the benefit is better memory bandwidth can increase the frame rate is your favorite 3D game.