For those of you looking for some serious honk and a simple swap, the Stebel TM80 Magnum twins are only ~$26 shipped from
http://www.hornsplusauto.com/.
You can hear a sound clip of them at
http://www.stebel.it/stebel/index.htm.
I did the upgrade this weekend, and it only takes a few minutes:
1. Disconnect your battery.2. Remove stock disc horns from the brackets - 1 electrical connector and a 10mm nut holds them on.3. Install Stebel horns reusing the stock nut. You will probably have to bend your stock brackets a bit if you have A/C. Make sure the horns are pointing down.4. Make a ground wire for each horn from an insulated female butt connector and some decent gauge wire. You can ground the horns on the nut right next to your headlights (look over about 6" from the side of the horn). Be sure to coat the inside of the connectors with dielectric grease.5. Hook up wiring according to the diagram on the horn packaging.6. Reconnect your battery and test the horns.
I put Hella Supertones on one of my B13s; they were expensive, obnoxiously loud, and had to be wired directly to the battery. The Magnums are cheap, very loud but not ridiculous, and the stock wiring should be able to handle them.
You will be heard with these horns. I need them in DC because Maryland drivers are terrible, and the cabbies here are brainless.