You need to know the size of your individual pictures. If you are using a fairly new camera on maximum resolution, ie highest quality, then you will typically get picture sizes of about 1MB, perhaps a bit more, or a bit less. Therefore you would expect 64 pictures off a 64MB card, plus the existing memory in the camera. Pro rata for 128MB card etc. If you are only after lower resolution pictures, ie for postcard size printing or uploading onto computer, then you will be more than happy with image sizes of less than 250MB, which will increase your capacity fourfold.
I hope this helps ...
Derekthefox :D