engine boring!

I was thinking of boring out my 400, but I don't know what all the numbers mean. Like if you go .20 or .40 over, what does that make the new bore. And are big bore kits really worth it? I searched for it in the forum, but couldn't really find the answer I needed.
Thanks in advance for the replies.

P.S. just installed the extreme disc brakes, and wow what a difference! Love em.