Besides the sternum and the upper ribs, there are two more ideal targets for body shots.

1.) the Liver: Probably the one spot on the body that striking could result in a one-punch KO. It's on the right side of your body, just above the hip-bone and right below the lowest rib (so for you orthadox fighters, that means throw the left hook to the body. For southpaws, through the straight left). It is an ideal target for several reasons. Firstly, it is a major organ that is completely exposed (i.e. not protected by bone). And second, if you hit it just right, it will (for all intents and purposes) paralyze your opponent. I'm sure some of you reading this know what I'm talking about. You can't breath, you can't feel your legs, all you want to do is curl up in a fetal position, while wishing that someone would just put you out of your misery because it hurts so damn bad. The liver is a perfect target. DeLaHoya was KO'd by Hopkins via a liver punch; Gatti KO'd Dorin with a liver shot; there are a ton of examples out there, and it happens far more frequently than people often admit. Probably the best example of a fighter with a tremendous liver punch, is Irish Mickey Ward. He dispatched many opponents with this punch (even dropping Gatti with it, in the first fight of their trilogy). Watch Ward fights, and you will see how to throw an effective liver punch.

2.) The floating ribs: The last three ribs on the bottom of your rib cage, do not connect to your sternum the way all of your other ribs do. They just kind of float there (hence the name). Because of this, they are rather vulnerable. Check out an anatomy book, and you'll see what I'm talking about. You probably won't KO anyone with a shot here, but they will feel it. I promise. It hurts a great deal to get banged in the floating ribs. Hit hard enough, you'll probably break something in there.

Hope that helps!