[Glorfinniel the Lost]

The Istari were sent by the Valar in the guise of mortal men, so as to gain friendship and trust, rather than coming in their full might and majesty which would have daunted men, and seem that the wizards had come to conquer and rule.
The Valar did the opposite of this by attempting to gaurd and seclude the elves with an open display of power,which Melkor used against them, leading to the exodus of the Noldor.

My point? Power, might and courage can come in the simplest or least expected form. Take Frodo and Boromir for example. Frodo, a simple hobbit entrusted with the task of destroying the One Ring and for the greater part, aided only by Sam. And Boromir, a valiant prince of Gondor, honoured and respected by all, yet who resisted the temptation of the ring and destroyed it?(even if Gollum bit it off his finger in the end, Frodo at many times could have surrendered to Sauron or turned back on the quest)

In Gandalfs case, power came in the simple form of trickery and cunning. He lured the Balrog onto the bridge, and an unsuspecting Balrog went bye bye when Gandalf broke the bridge with his staff. But the Balrogs whip took hold of Gandalf, bringing him down to the depths of Moria.
In the end, Gandalf did defeat the Balrog, but became unconcious (during this time, I believe his spirit went back to Valinor for some unknown reason, but he awoke Gandalf the White.)

And finally, the point of this whole post being? Gandalf probaly knew that by "dying" he would receive a lot more power; this power being vital in many battles later in the books.
But this as Guildo puts it, is just my two lembas and someone with more knowledge about subjects like this might like to clear this up?