Laptops

I'm looking into getting a laptop for my work trip to Chicago. I found a pretty sweet Pentium 4 HT at Best Buy but the battery life is only about 2 hours. Now I'm thinking about a Centrino since they have a longer battery life. There are some chargers/battery adaptors at Best Buy that claim to give your laptop an extra 5 hours of battery life. Have you heard of this? Does it work?