Here is a little chart that may help. It only goes up to 6 megapixel but it may be good enough.
http://www.digitalcactus.co.uk/memorycards.cfm
So basically you need to find out what megapixel size your camera is.
As you can see, for a 2mp camera a 16Mb card will hold about 18 pictures, and a 128Mb card will hold about 145 pictures.
The more megapixels the camera is the less pictures a memory card will hold.
These figures are only a rought guide because each picture you take is slightly a different size.