You want as many of each as possible, and you probably want them in roughly equal numbers. It is my experience that dredging one card a turn is not fast enough in most matchups to win. I would not keep a 7 card hand with a dredger, discard outlet (and mana) but no way to draw cards. I also wouldn't keep Land, Land, Troll, Breakthrough, Breakthrough, 2 blanks (although I would keep it if it had a Petal, even in place of one of the lands).