I have a Sony 1 gig and a SD 2 gig in the 505. I have 2122 books stored. It took over 20 minutes for the initial sorting but now it access's the books fairly quickly.
Do I need all those books on two cards, nope, but I put them on there to kind of push the limit, and I have not found it yet. I travel internationally and I am an avid reader so I don't like getting caught with out plenty of material while on those long plane rides and waits in the terminals.