If you plan on making a lot of all-motor power, get some kind of 2.5" header, a Carsound 2.5" cat that you'll need to have flanges welded onto by a muffler shop, and a custom 2.5" catback.

The stock Honda JDM 98spec ITR header, DC Sports JDM 4-1, JG/Edelbrock, Spoon, Mugen, Toda, SMSP and Hytech headers all have a 2.5" collector.

If you're just looking to do intake/header/exhaust just get the bolt ons and keep your stock cat. It's a helluva lot simpler.