Memory cards vary greatly in their storage capacity and transfer speeds. If you do not have memory cards, you should consider the different types of cards available to determine the best one for your needs. If you already have memory cards and are looking for a compatible reader, look for one with sufficient slots that is also compatible with your computer. There are two ways you should go about choosing a memory card reader. There are dozens of types of memory cards, but many memory card readers are compatible with dozens of different card formats.