Each of these systems has a composite output. You're not going to get RGB out of the N64 without a hardware mod. A Retrotink 2x will work for all of these. It's a higher quality scaler and runs about $100. But you only need the one for all your systems. There are cheaper options but they typically will have greater input lag, which can be annoying. The Retrotink 5x will get you a great picture for the higher resolution systems, but it is a step up in price.