Lexar is one of the best, if not THE best. I know CF cards are rated for data transfer speed, but I'm not sure about SD cards. If they are it's best to get the fastest you can afford. However, keep in mind that your camera can only read and write to the card at a certain top speed. If you were to get a 40x card, but your camera couldn't read/write to it that fast, then you'd almost be wasting your money. However, the difference would be made up when you upload the images to your computer, assuming your reader can take advantage of such speeds, such as the reader I mentioned in another of your posts.
In memory of Christopher James Roy, killed in action, November 28, 2005, and Eric Harless, taken by cancer April 29, 2006. We hardly knew you. May you each rest in peace.