Here's everything you need to know about credit scores and how they are calculated:
http://www.myfico.com/CreditEducation/WhatsInYourScore.aspx
And here's another good reference site - Clark Howard is the man. Somewhere in there he references to make sure you are not using more than 1/3 of your available credit, as that can cause a big hit on your credit rating.
http://clarkhoward.com/library/tips/credit_scores.html