Yes, chickens can be a good help in the garden providing you live in an enlightened area that allows you to keep some. If you live in the typical sundivision where the houses are all made of tickey tacky and are all in a row and must maintain a neatly mowed, "weed" free, lawn you may not be able to.

I understand the restrictions of the typical subdivision. If you live in a community that is more relaxed about having chickens, having 3-4 for a typical-sized yard can be really awesome though. In Paul Wheaton's podcast on methods for raising chickens, he talks about the pros and cons of different management systems. While some of the info is more helpful to people specifically focused on raising chickens, there is a lot of good info spread throughout that is good for anyone raising any number of chickens on any size property.

Here is a link that might be useful: Chicken podcast