As I described in
this thread, I tried to load the 10,699 mobipocket books on the blackmask dvd onto a 4gb memory card.
I wanted to know how much the Kindle could hold without crashing. I'll share my experience with you. The Kindle usually wimped out at around 700 pages of books in the Home screen (12 items per page). By wimp out, I mean that the display, the buttons, and the scroll wheel froze. On a few of my attempts, the Kindle recognized up to 780 pages before freezing, but that was the minority case. Also, it became very sluggish. I noticed that, if the list grew that large, my books on the Kindle's memory stopped being displayed. Based on my experience, I'd say that you could keep 5000 books in the documents folder of an sd card without experiencing a crash. This is just a conservative guess. The limitation on the number of files may be something that disappears in the future should Amazon allow folders. Who knows.