no replacement for displacement????

Been doing some reading here and there, what is the deal with gear ratios with forced induction? I am unsure of what I will be doing, but I need to get axles and a center case for my car before I go to get it. I suppose I could hold off on the center case til later...but still wondering about gearing of a forced induction vehicle. I knew driving that diesel truck with a turbo was going to get me hooked on it....