Yuckysgf: CAS2 is maximally 33% faster than CAS3, but you'll never see this in real-world performance, as the system bottleneck is almost never solely the memory timings. Honestly, work out the math yourself: you have at least 2+N clock cycles per N transfers saved with CAS2 over CAS3. A single CAS2 transfer takes 4+2N clock cycles (I think! someone correct me here if I'm wrong) and a single CAS3 transfer takes 6+3N clock cycles. CAS2, then, will be maximally 50% faster (6 clock cycles vs. 9 clock cycles) over CAS3. In an infinite length transfer, CAS2 will be 33% faster than CAS3.
Bottom line: If you commonly do tasks which stress all aspects of the computer (games, office apps) then yes, CAS isn't that important - 5%, maybe 10% at best. However, if you commonly do tasks which stress the memory bus (numerical simulations, or 3D rendering on a system with huge amounts of RAM - and not displaying it to screen) then you will see a massive difference between CAS2 and CAS3. Granted - a lot of people in here don't do numerical sims or massive 3D rendering, but I just wanted to remind all of the techies out there, if you recommend a computer system to a friend who works in engineering/science fields, try to maximize the memory and cache speeds.