dpi and inches won't matter as long as your card image ends up at 1500 x 2100 pixels. I scan my artwork for a variety of uses and always leave my scanner set at 800 dpi. For my cards I resize the image in increments until it gets to the correct size. You get a better quality if you size it down in steps of around 10% or so at a time. I guess if you are only using your art for cards then you could save some work by scanning at a resolution that would end up fitting into your 1500 x 2100 pixels card design. I hope that is some help.