My God must be more than a product of my imagination. Phil, one will never know God, if He is reduced to some sort of test-able hypothesis. Indeed, the fact that He can't be known in that sense, speaks to the veracity of His existence. As for a god being neccessary for spirituality, I would say no.
Spirituality is an inately human trait.
Although it is often reduced to the rank of evolutionary artifact, it is nonetheless real in the sense that we are all born with it.