The max size of the file that your camera creates even if you shoot at highest resolution is just under 3 MB, therefore buying a 2 GB card just does not make any sense as a 2 GB card will accommodate almost 700 pictures.
Cameras which have the option of TIFF or RAW formats with file sizes ranging from 5 to 20 MB could do better with large capacity cards as opposed to yours which only does JPEG. Keeping that many pictures in the card without transferring them and not ensuring that they are lost because of card failure, is not something I would be comfortable with. You have to think of how many pictures you would take in a single session or in a day. I doubt anyone can take 700 pictures in a single session or even a day. Besides if the picture is bad you could always delete them in the camera.
If I were you I would go for something like 512 MB card, maybe two of them for the Pentax S4