Easy answer, gearing. I put a 4.10 OS Giken diff in mine, and LOVE it.
More thoughtful answer, all depends on what you're using the car for. Set goals, then modify to suit. If you're doing autocross or drifting, a fairly small turbo that favors response will be your best choice. Drag racing, road racing, quick street car, highway monster... what do you want to do with the car?