if you ad horns you are going to want some sort of a processor so a new active capable headunit is your best bet there. then you will want to find the horns you want, and a nice set of mids. feed them some decent power and they should sound pretty good.
there is more to it than that but you should get the idea
to properly run horns you are going to be dropping a pretty good penny, hence why i never have been able to run them //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif