Cosmic Cube
I don't have any friends.
Here's a pretty good explaination of the Hulk's power:
First off, exposure to mass amounts of gamma radiation should be fatal. If for some reason, gamma radiaton doesn't kill you, it acts as an agent that allows whatever is in the deepest parts of your brain to somehow take physical form within the physical realm. Exactly how it would do so is a mystery, but the power likely comes from an extradimensional source. Doc Samson drew this theory from the study of several victims of gamma ray exposure.
Jen Walters always had a low self image, and wanted to be the perfect female specimen, beautiful and attractive. She gets gamma ray infused blood from he cousin, and you've got the She Hulk. Samuel Sterns was a borderline retard, he had always dreamt of being highly intelligent. He gets hit with gamma radiation and becomes the Leader. Emil Blonsky was a russian spy, who inwardly thought of himself as horrible monster. He gets hit by gamma radiation becomes the Abomination. Doctor Leonard Samson himself had always dreamed of being physically impressive and heroic like his biblical namesake. He gets radiation and you've got Doc Samson, who's strength, like his namesake's, is relative to the length of his hair. In the Incredible Hulk #250, Surfer dreamed of being free of the barrier Galactus used to confine him to the earth. After absorbing the Hulk's gamma radiation, Surfer was able to break free of Galactus's barrier. The list goes on.
Bruce Banner, is a bit more complicated. He and Jennifer Walters are the only two gamma radiation victims who can revert to their human forms. As a child, Bruce and his mother were abused by his father. These traumatic experiences caused him to develop a Multiple Personality Disorder. Bruce felt completely powerless, and unable to defend himself or his loved ones from his father's fury. In his mind, he created an imaginary friend, modeled after one of his favorite toys, and named it the Hulk. One who would always be strong enough to protect him, and the ones he loved. Once Banner was exposed to the gamma radiation, from some extraphysical source, the Hulk was given physical form.
The Hulk is basically an imaginary bieng, come to reality via gamma radiation. As a product of the imagination, (that of a child, no less,) his strength isn't limited by the laws of the physical universe. There is no actual limit to how strong he can become. The Hulk's strength is intimatley tied to his anger. As the old oddage goes, 'the madder Hulk gets, the stronger Hulk gets.' There are two reasons for this. Firstly, being too weak to defend himself, or his mother, made young Bruce very angry. Secondly, the Banner persona restrains the Hulk's power. Bruce doesn't really want to hurt anyone, or become the monster that his father was. As the Hulk becomes angrier, Banner loses more and more of his control over the Hulk, thus releasing more and more of the Hulk's limitless power. Though it seems that Mindless Hulk would be the strongest incarnation, the paradox of his power is that the Mindless Hulk in a way acts to limit himself. Anger is required for greater strength, but, if Banner is completley removed, so is the necessary brain functions for higher emotional stimilus. For example, the Hulk can't get too mad about a villian killing Betty, if he doesn't know or care who Betty is. In a way, Banner is both the source of the anger, as well as the controling influence of the anger. Most of this is explained further in The Incredible Hulk #393.
The Hulk's strength potential would be whatever one might imagine. In the Marvel Universe, this would place the Hulk's potential in the upper cosmic realm. But for the most part, he's just operates at the Superman/Thor strength level. Top tier hero status.