And do not underestimate the importance of a good charger with your batteries.
I'm sure one of the engineering types around here can explain the details, but this one in the link below comes highly recommended in several other photo boards I read. I have Syl Arena's book the "Speedliter's Handbook" and he recommends the 8 slot version. I have this one, the 4 slot version, and it is great. It charges each battery individually (not all chargers do that), plus has specialized modes depending on if you are breaking in new batteries or trying to refresh really old ones.
http://www.amazon.com/Powerex-MH-C9000-PowerEx-WizardOne-Charger-Analyzer/dp/B000NLUSLM