I'll put it this way, a perfect penalty is unstoppable, no matter how good the goalie reads/guesses. These are typically characterized by being in the upper half of the net, with pace, and generally hitting the corner or side netting. I want to say there was a German study that looked at PKs and found that when players aim for the top half of the net, and their shot is on target, they score something along the lines of 98% of their penalties. Of course, going this route, you run the risk of becoming the next Roberto Baggio.
A good to great penalty is only stoppable when a goalie both reads/guesses correctly, has the athletic ability/agility to get in the proper position, AND is strong enough to keep it out of the net. This is what happened on the 2nd PK stop for Romero. It was a tremendous save. Throw in that it basically sealed a spot in the World Cup final, and it makes it even prettier.
Only a mediocre penalty can be stopped by luck alone. This is typically how penalties are stopped (including how Romero stopped the first PK). The goalie guesses, and the player hits a little low shot that basically hits the goalie (as opposed to the goalie actually making the stop). I would never praise a goalie for making this type of save. It's routine, and stops about 1 in 5 or 6 penalties because the players get too nervous to try and hit a proper penalty.
