I've been using audible.com to download books for 6-7 years and have used different MP3 players along the way.
I currently have a 1G Sandisk that I use the most. I can fit almost 20 books on it in audible format 2. I listen to a set of books, delete them (they are still on my computer and in my audible library online so are not lost), and then load some more. Hours and hours and hours of books can fit in 1G.
Most important is whether the MP3 will support book format - many, many do not. I use audible format and often have to order my MP3 online to get what I want.
Some other things to think about. One with batteries (usually
AAA) will chew up the batteries pretty quickly but it is worth it if you are out of the country or on a long plane flight where you cannot recharge the batteries.
I have a Zen Stone with a rechargeable internal battery that is very tiny and cute but the battery life is probably less than 10 hours. If I fall asleep with it on (it has no sleep feature) it will be dead the next day and takes a long time to recharge. I find it needs to be recharged almost daily.
My Sandisk Sansa uses 2 AAA batteries - probably about every 5-6 days. It helps to use rechargeable batteries - plus I can buy AAA batteries everywhere (which I did in Italy last year).
Some have a little screen and a fairly extensive menu. My Stone does not - just turn it on or off and adjust the volume. I think there's a newer one with more features.