Hi !!! I went to a retailer who showed me 3 different SD memory cards from Sandisk. All were 1 GB. The major difference lied on their "speed" so the salesman said. One model was 2 (of speed), the other 9 (of speed) (the ultra II model), and the last one 15 (of speed) I think (the extreme model). My question is what is that about speed that I didn´t really understood or may be the salesman didn´t explain me correctly. I intend to use the card with a Nikon D50 camera, which would be the best card for me ??? Thank you very much, igmolinav.