Memory cards should be purchased according to how high the resolution or DPI (dots per inch) of your digital camera is (i.e. how many megapixels it has), and how many pictures you plan on taking at a time. Transcend have provided the following graph to assist you in buying an appropriate capacity card for your camera:

*The "File size" represented in the graph below assumes that the highest resolution (DPI) your camera supports is used and that pictures are stored in standard JPEG format. 
The actual number of pictures you can take will vary depending on your digital camera's resolution and compression settings, composition (complexity) of the images taken, as well as lighting and exposure factors.