If you aren't hitting as big a serve as you should be, the culprit almost always has to do with not properly setting up - and executing - the pronation move. When examining a player's first serve, the first position I look for is the "Big L" position. In the final frame in the Nalbandian video, notice how Nalbandian's arm is completely straight, while the butt cap of his racket faces the net and his strings face the side fence.
This big "L" position allows you to twist (rotate) the hand and arm (while leveling the wrist with the forearm) to generate a powerful torque into the ball.
In the bottom video, watch pronation in action. Nalbandian does not "snap" his wrist. Instead, he turns his hand and entire arm into contact, generating a torque into the ball. The wrist "catches up" to the forearm during this twisting motion - it goes from a 90 degree angle with the forearm in the first frame to level with the forearm in the second frame.
The final stage of pronation is where the arm and racket continue to extend out towards the net together. The hand, wrist and arm form a "line", as a unit, extending towards the net. If you snap your wrist forward, you lose this "line".
The two clips on the right are all that are needed to hit a monster overhead. Just get into the "big L" and pronate.