Don't just stop with the header. A more free flowing exhaust past the tailpipe will help. Basically a full 2.5 inch exhaust would help. I'm not sure about smog and emissions standards in wisconson, but removing the catalitic converter will make it a little more free flowing thus freeing up some more power. Usually run about $250 up. -Other simple things include a K&N air filter (much better flowing. 3-10 horsepower gains.) $45 usually. -underdrive pulley. basically it's a replacement for your stock crankshaft/accessory drive pully. It'll spin all the accessories (alternator, a/c, power steering) faster than needed, which will give your car about 5 more hp. $70 - $110 -removing the a/c compressor. Since you're in wisconson, I don't imagine it gets too unreasonably hot there that you couldn't survive without air conditioning. Doing this will free up some horsepower that is wasted in rotating the compressor. Also for a little bit of weight reduction, but not much. Somewhere around 5-10 horsepower could be expected. But if you were to do this though, you'll first have to get it drained by a professional Just a few pointers
Good luck