Our library gives us a 250 book limit per card. I love it! The library that we go to is great and all the employees are really nice to the kids, even though they get a little too loud at times.
I only have a card for me because my boys are too young to keep track of their due dates. Once they are more comfortable using a calendar, I'll let them each get a card.
Right now I'm in the process of ordering books to last us a couple weeks. Until just a little bit before school started, I thought I was going to send my oldest to public school. So I didn't prepare at all.
As for overworking the library, I don't think so. The library gets money directly based on their circulation and we contribute to that big time.