To add to the CF debate, I use Ultra II's and the only time I really feel the pinch in speed is car races and air shows. That is with a 22 frame RAW buffer at 5fps. With the 400D you have a 27 frame buffer for JPG and 10 for RAW, at 3 fps. That's not bad, you'll just have to keep your bursts down to 5 at a time if you shoot sports. That will let the buffer do it's job without it getting totaly full and having to wait for it to write the card. I carry 2x4gb, 1x2gb and 3x1gb and routinely fill them all up. I bracket a lot and shoot a lot, so that's me. A 4gb card should hold you until you know how much you are going to be shooting. But at 35GBP, I would get another 4gb.