The PSP games are prequels to the God of War series. They take place before the events of the first game, where they're based more around Kratos' relatives. However, God of War: Ascension will also be a prequel; it's just that this is a prequel being handled by Sony Santa Monica (basically, that and the multiplayer are their selling points).
That's not accurate because unlike Chains of Olympus, Ghost of Sparta takes place between GoW1 and GoW2, so only one is a prequel to the whole series.
Chronilogically: Ascension > CoO > GoW1 > GoS > GoW2 > GoW3