Classica, I think you'll be ok even with large books, as long as they are LRF format and not super image heavy. I've got a combined ebook page amount of 50,000 pages on my Reader (spread over 80 books, and an ebook page is *usually* about double the amount you'd see on a pbook, but not always) and I still have half the memory left, just to give you an idea.
Not trying to talk you into it, just trying to help you make an informed decision
