Most importantly with the D80, you get a MUCH better autofocus system (the 11 point system is FAR superior to the 3 focus points even if you only just use one focus spot... you can move it around a lot more) and a vastly superior image processor... You also get a built in commander mode for so that you can use the nikon flash system to remotely fire several flashes, you get depth of field preview (which is IMHO essential for any serious photography work), you get a more robust shutter, you get the ability to use FP flash mode (basically use flash at high shutter speed... a big advantage the D80 has over the D40x... surprisingly, the simple D40 is better than both with its native 1/500th of a second flash sync speed), and you get a top LCD so you don't have to just rely on the back LCD for your camera info. The histogram is better too.