7. Why are so many Drupal versions available - 4.x, 5.x ...? Which one should I use?
It is recommended that you run the most current stable release. This can always be found at the Drupal Project page. However, if there are no compelling features in the latest version, a contrib module that is important to you isn't ready or you don't have time, there is no need to rush your upgrade as long as security updates are available for the version you are running.