Yep. the lower the number the better. it basically says how many clock cycles memory takes to do a job. If one type of memory is 400mhz and takes two cycles to do a job, that means it took 5 nanoseconds. If another type of memory is 800mhz and takes four cycles to a job, it'll also take 5 nanoseconds. However, with regard to overclocking, higher mhz is better, so the 800mhz CAS4 would be better than 400mhz CAS2.