Really depends on your choice and wallet.
There's much more knowledgable folks on here, but as I recall you have 3 options for big power gains...

1)Throttle bodies combined with other NA Tuning such as uprated cams, lightened flywheel and pulleys.

2)Turbocharger plus the essential head work/pistons etc.

3)Supercharger plus the previously mentioned NA Tuning.

Like I said, its your choice and wallet, obviously NOS can be used but I know little about it.