How many pictures will a 320 gb hard drive hold?

A memory card with a capacity C will hold N images approximately, where N = (C-O)/A. O is an overhead figure, accounting for storing names of pictures and maintaining the directory structures, and A is the average size per image. The overhead figure can be ignored, as it is small compared to the totol capacity C and the variation of A. The average image size is a function of the camera's resolution and the subject depicted, and can be anything between several tens of kilobytes to several tens of megabytes. The image size varies with the subject because different subjects compress differently; a photo of a plain white wall will need less storage than that of complex structures in a flower bed, for example. Assuming an average image size of 12 MB -a pretty high value only needed with slightly over-average high-resolution cameras-, one can estimate N as 160GB * 1024MB/GB / 12 MB/image = 13,653 images. For currently available commercial cameras and an average spread of subjects, it is probably safe to say that a 160GB memory part can store at least 14,000 images. Your mileage may vary though.