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Turbo, resonator, cat, muffler.... in that order are what affect the sound the most. Pipe diameter affects both pitch and volume but isn't something you should screw with much if you're worried about performance and you car is mostly stock (and there's not a single upgrade you can make on a GE to justify larger than 2.5", turbo'd engines are a whole different ball game).

Turbos, cats, resonators and mufflers all smooth out the oscillations in your cars exhaust. The exhaust exits the header as a continuous series of little bubbles. The pockets of lower pressure and then the jump back up are what cause that wicked lovely ricer fart sound... there's no performance there, just someone with no clue how it works or is looking to be loud and obvious for the cops just for the sake of it.

Now there are different and better types muffler and resonator... the stock cat flows VERY well, but you can get even better flowing ones pretty cheap. As for mufflers and resonators you want to look for free flowing but from a company with a clue... I like Borla's ProXS series and Magnaflow, but I find I prefer the sound of the Borla most. Both are straight through mufflers, to level the tone more and reduce at idle and low load volume a dual outlet model is preferred. With resonators you'll be hard pressed to find a straight through model. There is one made by AeroTurbo which is a straight through, helmholtz resonator... the only way to go. Smooth deep sound with an increase in exhaust velocity.

One more thing you can do to help the exhaust system is to increase the pipe size after the resonator by about 1/4" this provides extra volume to allow for the increasing density of the cooling exhaust gasses.... but don't jump up in size too quickly. The ideal pipe diameter is actually a very small and steady increase in size... a VERY long narrow cone. Kind of hard to get that fabricated, but you can use a couple small steps up in size as the pipe gets closer toward the back of the car.

No headers have very little affect on sound.... at least on a car with such a long exhaust track, any interruptions and piping anywhere near the stock diameter. Don't buy headers for sound.... don't buy any upgrade just for sound, it's a waste of money, IMO.