The image scanned into the PC should ALWAYS be significantly larger than the 1024-wide A.net standard. This allows you to rotate and crop the picture to be exactly what you want, before resizing it to 1024 pixels across. I would say that 1500 pixels wide for the initial scan is the minimum, although I usually scan at 3000-4000 wide.
If you do upload a picture larger than 1024, A.net's scripts automatically resize it down, but there will almost certainly be a loss of picture quality. Better to resize it yourself to the proper size.
640x480 is too small for this web site. The minimum is 800 pixels across, for a landscape-format photo. For your digital camera, I would set the image size to 1280 across, and even more if you can manage it, so that you can play with the image before resizing it.
About your laptop. Don't do it. When I am using my laptop, I never screen other people's pictures, because I know that the laptop's definition (even though supposedly at 24-bit color and 1024x768 definition) is not as good as a desktop screen, and a change of viewing angle of just a couple of degrees causes the image to look much different. Nor do I ever try to process my own pictures using a laptop screen.