I have the first iPad
, and I got the 16gb, thinking as i have a 16gb iPhone it would be enough. I was so very wrong. Along with all the apps, music, photos and small amount of video I am constantly getting the message to delete something. You may be different, but in the long run I honestly think it is worth shelling out the extra cash for the 32gb, if not the 64.
Hope this helps.
Edit: oh an don't get iBooks. Kindle all the way. iBooks is more expensive and has a much smaller range of books available!