RE: Goldberg Eagle 400 motor/battery?
Whether you realize it or not, you're well on your way to becoming an electric expert. Yes indeed, a battery with more cells (higher voltage) will improve power, and quite frankly, I wouldn't run any fewer than 8 cells. 9 might shorten the life of your motor drastically, but that's okay because brushless motors and ESCs are getting downright cheap these days.
With something like an Eagle 400, you probably don't need brushless right away, and you certainly don't need LiPoly. This is a cruising plane, not an aerobatic or 3D rocket ship, and cruising doesn't require a whole lot of power.