I'd say you upgrade the cargobay on your Sidewinder to 6 tons, go with it until you can afford the Hauler (can easily get 12-16 tons of cargo).
Hauler would be your next step after an Eagle. I don't recall exactly the number, but I think the Eagle has the same cargo limitations as the Sidewinder, hence it's not suited for trading - it's a money sink if you aim for the Hauler.
PS, any money you put into upgrades should be payed back when you trade in the ship at the dealer - so upgrading is not a waste. But they do deduct some 10% off the market value when you trade in your stuff.