From my experience, the speed difference between cl4 and cl5 is not much. It used to be back in the days of simms and dimms, but as soon as DDR2 ram showed up, the difference shrank. You still want the fastest you can afford, but its not so much a difference between cl4 and cl5 as it is a difference between cl4 and cl7 or greater. Again, that's my real world experience. I'm sure that specific tests may show theoretical differences, but are you really going to notice a few microseconds in your everyday computing? I doubt it, or rather, I don't.