Hopefully one of the paint experts will answer but on previously painted walls I just clean (with TSP, Simple Green, or ammonia solution), spackle and start painting. If the paint is semi-gloss or enamel (kitchen or bath paint) I use primer or when going to a real light color from a darker color. As far as product preferences - I use Behr or Glidden. Here's a paint link that may help too. Check out Glidden's website as well.
http://www.bobvila.com/GlobalFrameSet/index.html?/FAQ/FAQ_Paint.html
Good luck.