NA == naturally aspirated, which is "normal" for the majority of the cars on the road.

FI == forced induction, which is either a turbo or supercharger. It forces air into the engine, generating *much* more horsepower.

The problem is that the good engine mods for an NA car are counter productive to one that has FI, so you have to think about the investment. (you'd have to replace prettymuch everything)