The Sagas will start after Garilc Jr. is destroyed on Earth and the first Player arrives on Namek.
Frieza will be fought when the necessary Namek dragonballs are found. After he is defeated at Namek the rest of the events will unfold.
Cyborg Frieza (2 weeks after he is defeated at Namek)
King Kold (1 week after Cyborg Frieza)
Androids 19 and 20 (4 weeks after King Kold)
Androids 17 and 18 and Cell (1 week after #19 and #20)