Lots more questions need to be answered, but...
What kind of engine are we talking about here? What is the operating range of the valvetrain? What kind of gearing and tiresize?
Generally 2.25" dual exhaust is good for a 350hp motor, 2.5" is 500hp. Unless you are making above 350hp there is no reason to buy a new exhaust IMO.
If you get headers, don't buy cheap ones that will end up leaking on you, and then factor if headers are worth the 1-2mpg you may gain?
What kind of MPG are you getting now?