If you're going to use it for audiobooks downloaded from the internet, and if you're going to use it for a site like Audible.com, I would go on their site and take a look at their list of compatible players. Audible uses a special format (NOT mp3) that is not compatible with all players. As for capacity, it depends on how many you want to load at once. For books, I doubt you'd need something huge and could probably stick to the 1-2 GB range, as you'd probably be changing them out frequently anyway. A long book could be up to 5-6 CDs in length, and a 2GB player could hold 3 or 4 books at a time, at least (although I don't know the compression on Audible's format). (This means you wouldn't have to spend an arm and leg on huge capacity, either) The Creative Zen Stone is compatible with Audible, 1 GB is only about $40. So is the new Sansa Express, and it has a built in standard USB plug (a departure from the norm for Sansa), 1 GB at about $60 (with micro SD card slot for more memory).
I am planning to use an mp3 primarily for recorded books or books downloaded from the internet. Do i need any particular requirements or capabilities? How much memory?