Since the Reader has a flat file structure, a good question is "how many titles do you want on a memory card?" From what you say, sfernald, 1500 titles is too many, and I agree with you on that. I think the optimum size is between 100 and 200 titles, whoch is 10-20 pages in the Books menu. Multiply that by the average size of file, and that gives you the desired capacity of the card. If you're reading short stories, each story takes less than 100k, so that's 20-40 MB (at 200 titles). Manga, however, takes tens of MB per story, so a 1 GB memory card would hold less than 100 manga books. For that reason, 2 GB seems like a reasonable upper limit for memory cards.
(What can I say, I like doing ballpark calculations.
All of what I said will become moot when the Reader is upgraded to handle folders.