tgi asked someone to explain the difference between cas 2 and cas 3.
Cas 2 is faster, benchmarks have shown it.
at crucial.com it says:
"CL=2" (also written "CL2" or "CAS=2"

refers to a module's CAS latency. CAS latency is the amount of time it takes for your memory to respond to a command. It only affects the initial burst of data. Once data starts flowing, latency has no effect.
Latency is measured in terms of clock cycles. A CL=2 part requires two clock cycles to respond, and a CL=3 part requires three clock cycles. Thus, CL=2 parts complete the initial data access a little more quickly than CL=3 parts. However, a clock cycle for a systems with a 100MHz front side bus is only 10 nanoseconds (10 billionths of a second), so you probably won't be able to tell the difference between a CL=2 and a CL=3 part.
Most systems will accept either part; however, some systems require one or the other. These requirements are built into our Memory Selector."