Bar-en-Danwedh
House of Ransom
i beleive Gandalf also says in the book at the counsel of Elrond that the power available to the wearer was subject to the wearers power and skills...
the average punter to put it on was only able to take advantage of its invisibility feature and even then it was without the ability to controll it...
Tom Bombadil was powerful enough to wear it without entering the void... and even then powerfull enough to see frodo invisible...
the ring could not be controlled as such by any man... because part of saurons will and spirit were in it... Wizards would have been a slightly deifferent case... they were far more powerful and would have had an infinatley wider set of uses for it. Gandalf or Saruman, WOULD have been able to use, but it would be their own will that they would have lost controll over... if Gandalf had taken the ring he would have had a whole new array of abilities at his disposal, and knowing the location of the three elven rings he would been able to control them, And already having one of those rings would have been an added bonus, he probably would have been able to control the ringwraiths, but as his will would be lost to sauron, nothing really woiuld change for the ringwraiths... i think Saruman would have had a better chance to control the nazgul, then gandalf because of his voice...
but in the end, Gandalf would have eventually brought the ring to sauron... who would reclaim it and make Gandalf his slave...
a high ranking one though...