The dpi (dots per inch) is relevant for printing but not much else. You can alter dpi at will for your images, but you'll want to avoid resampling. So, set the Image Size in CS3, and make sure the box to Resample Image is UNchecked. Then, when you increase the dpi using the Resolution field (with the pop-up menu set to pixel/inch) to 300, the Width and Height will change, but the pixel dimensions of the image will not, preserving your image quality.