I agree that it's not totally obvious to new customers, but it's also no that hard: within a generation, the relative performance of the cards corresponds to the order of model numbers. "ti" cards are more powerful than non-"ti".

Between generations, the whole line moves up something like one level of performance, so an (X)70 should be compared against an (X-1)80 and so on.

It's not the simplest thing, but I think most people will do research once the first time they buy a GPU, and then you have your mental model from then on.