I also have a Fuji camera (4900). I'm not exactly sure what resolution the medium setting is, but when I am using the 1200x???? setting (sorry for the vaguness here) I can usually get approx. 100 pictures per 128 mb card. These pictures come out to about 1 mb per picture.
So using this logic, you should be able to get a minimum of 225 pictures per card.