ditto over here; 300 books and less then half full.
Edit: just checked: less then 1/4. The average converted pocket seems to take less then 0.25 MB. Real large (1000+ paper pages) less then 0.5 MB.
Considering the internal memory is 440MB and since I do not feel the need to carry around a full library I do not think the lack of a memory card is a dealbreaker