as usual, sheldon is a great source of info:
http://www.sheldonbrown.com/bbsize.html
and this took me exactly 15 seconds to find on shimano's site:
http://bike.shimano.com/publish/cont...inline.pdf.pdf
You may need to ask more questions (depending on how much you already know) about how to interpret the shimano site, but basically they are giving you the different configurations possible and the resulting chainline, which you will match up with your rear hub info to determine the proper bb.