how much flow in a 75 gallon tank

[saf]

I currently have a 75 gallon tank that I would like more flow in. I have a little giant pump in my sump that pumps about 1000 gallons with no head. It is plumbed to the main tank via two returns that run behind the rock wall, so there is not much agitation towards the front of the tank.

I was thinking of adding one or two seio 820's. Should I go with two or would that be too much flow? Would one suffice?

Thanks!