The most worth while part and money best spent was a TTI exhaust system for my cars. Use there mufflers, super turbos. My 440 with headers and a X pipe is really quite until I put my foot into it, but it's still not loud.
Personally I will always use H pipes in the future because the X pipe makes it almost too quite and sounds weird when you stand on it.
Checkout this short video of my 440 smoking the tires at 30 mPH roll for second, the motor is not even loud.
