I've been keeping a Mossberg 500 in my Explorer since July of `09. If the shit hits the fan, I have confidence in a .45 ACP to stop an assailant, I'm still going to want something more. a shotgun gives me a variety of rounds to defend myself and others, plus game shot and slugs for hunting.
There's a variety of carbines that would be ideal to compliment your sidearm and shotgun: 30/30, Hi Point's 9mm & .45, lever action .357's.... its a very large list. You might want to consider a carbine in the same caliber of as your carry weapon.
Cheaper Than Dirt has a decent article here: