Better to go all the way with intake header exhaust. Just by a twelve dollar intake from ebay and slap on a high flow, quality filter like a K&N, HKS, etc. For an exhaust you can by a catback from greddy, hks, apexi, etc. or you can by the muffler and have a shop fabricate the piping for you, (will save hundreds of dollars). Don't just slap a muffler on your stock piping . Also look into investing in a high flow cat. For headers you really won't see great gains unless you pay for a mugen, rmf, smsp, etc. Those can run u in excess of $1000. A cheap ($500) header will be more worthwhile if you were to slap a supercharger on there, but I don't know if they make SCs for the H23. With a $1000 budget you can do I/H/HFC/E, and that should give you a noticeable difference, not to mention if you do an oil change, clean the injectors, and replace the fuel filter, plugs, wires.

Doing all those things I mentioned above should give you better (3-5mpg) gas mileage because your throttle response will be better, well as long as you don't have a lead foot.