I believe that you can get window stickers for about $10 each. This will allow you to get 25 before tax. It is said to have a 10HP increase per sticker(headbang) . That's 250 HP. You would be the man.(rofl2) .
Just kidding. I would have to say the downpipe as well to remove certain constrictions with the exhaust.
are you wanting horsepower or are you wanting something that looks good? I think that the things on my car that have made me the happiest is the lowering springs, window tint, stripes and the kart boy shift bushings