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:

Storage Capacity	Camera Type
	2MP	3MP	4MP	5MP	6MP	8MP	10MP
4GB	4550	3412	2048	1638	1280	1048	816
8GB	9110	6824	4096	3276	2560	2096	1632
16GB	18200	13648	8192	6552	5120	4192	3264

*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.