Drupal 7 is in the RC phase, so it's time to begin thinking about your upgrade from 6 to 7. (My condolences if you're having to think about a 5 to 7 upgrade) I wish to present to you my thoughts on a new book which will prove rather helpful in upgrading your thinking to the new hotness of Drupal 7.
Drupal 7 is as different from Drupal 6 as that one was from Drupal 5. Making the upgrade seems to involve not just the technical stuff of the upgrade process, but also understanding what's new and how things are different. This book will give you a very good grounding in that newness.