How many pictures can an 8GB memory store?

It really depends on your camera and it's settings. The lower the resolution, and the higher the compression setting, the more pictures will fit on your card. That aside, the range is very large; it will fit between 1,200 and 62,000 depending on your setup. Pretty dramatic huh. And actually, if you were shooting on a Digital SLR or even on the new Canon Powershot G10, while shooting in RAW mode, it could be as little as 300 pictures. So as you can see, without details, it's hard to be precise in my answer.

The related link gives information on the maximum number of photos at varying megapixel ratings / varying image file size and for varying memory card capacities and may be of use.