Some good discussion here, and I'd like to throw in my $.02 worth on two points.
I don't think the caliber is the end all, though as said, I'd choose the heaviest available. Velocity by pretty much irrelevant; the pertinent parameters are either energy [1/2 mass * (velocity)^2] or momentum (mass * velocity). From the data I've found, .40 and .45 are pretty much a wash on comparison of either of these parameters, with muzzle energy of around 500 lb.ft in the hottest factory loads in either caliber. 9mm can push close to 400 lb.ft on +P loads, or about 350 lb.ft in regular loads. .380 ACP comes in around 200 lb.ft, along with .38 special. The big hitters are .44 mag ranging up to nearly 1000 lb.ft muzzle energy with the .357 mag up near 600 lb.ft.
Now compare to rifles, the wimpiest of which start around 1100-1200 lb.ft (.222) and are quickly up in the 2000+ lb.ft for even light deer rifles (.243, .25-06, etc.). Most of the popular deer rifles are in the 2500-3000 lb.ft range for muzzle energy.
Now, combine that with my personal experience that a deer shot in the chest at a distance of 100 yards with a rifle of say 1800 lb.ft (@ 100 yards) will typically run about 75 yards at full speed before expiring. These are small deer that are roughly the weight of a human.
I think if you combine all that, you come away with:
1. We're splitting hairs on pistol calibers--they're all pretty wimpy. There are differences, just saying we're arguing about 200 lb.ft vs. 500 lb.ft when 1800 lb.ft is not enough.
2. It's unlikely to be like the movies, and unlikely for the bad guy to get knocked back 5' and die instantly from a well placed pistol shot.
No matter, if you can't hit the target under extreme duress, it doesn't really matter. And very difficult to practice that.