Yes indeed that would be the best plan. Also refrain from using digital zoom as that can also be achieved on the computer when editing. In answer to your first question, Ken is correct about the quality affecting the size of the image file. Also the camera may have settings to save as different file types. Different file types have different compression algorithms. You might have seen what happens on your computer when you save a bitmap (.bmp) file as a jpeg (.jpg). The jpeg may be half or even smaller than the bmp file. Another attribute affecting file size is the complexity of the image. Take a photo of a clear blue sky and a photo of a heavily textured surface, say lawn. The image sizes will vary greatly due to the complexity of the image.
I have a 10 mega pixel camera and most photos are around 3mb in file size. On a 4gb card (which is 4,096mb) I could fit 1365 images.