What power did Sauron really have?

[Amarie of the Vanyar]

Sauron was already a necromancer in the First Age, as it is told in the Lay of Leithian [img]smilies/wink.gif[/img]

Not yet by Men enthralled adored,
now was he Morgoth's mightiest lord,
Master of Wolves, whose shivering howl
for ever echoed in the hills, and foul
enchantments and dark sigaldry
did weave and wield. In glamoury
that necromancer held his hosts
of phantoms and of wandering ghosts,
of misbegotten or spell-wronged
monsters that about him thronged,
working his bidding dark and vile:
the werewolves of the Wizard's Isle.
The Lay of Leithian; HoME 3

EDIT: Sorry, I missed your question about Gandalf [img]smilies/frown.gif[/img]

About his powers it is said that as Olorin his main power was:

though he loved Elves, he walked among them unseen, or in the form as one of them, and they did not know whence came the fair visions or the promptings of wisdom that he put into their hearts.
The Istari; Ufinished Tales

[ November 12, 2003: Message edited by: Amarie of the Vanyar ]