The Ramban asks an obvious question on this: What is so especially righteous about Abraham believing in the L-rd now? Here is a man who was in the process of undergoing ten trials given to him by G-d, including being thrown into a furnace, and going to war, and culminating in his willingness to sacrifice his son Isaac. Did that not show a tremendous amount of faith? What is so "righteous" about believing what G-d says, if He should come to you in a vision?
I would like to suggest an answer. What Abraham was displaying in passing his ten trials was not faith ["emuna"], but another character trait: heroism ["gevura"] -- the willingness to undergo any kind of hardship for G-d ["Mesiras nefesh"]. This is, of course, very important, and Abraham displayed this trait in full measure. But faith in G-d is another, perhaps greater quality: the ability to put yourself completely out of the picture when necessary, with the confidence that G-d will come through. (In fact, it does not say after any of the trials, that G-d counted it to him for righteousness!)
This was the case here. Although Abraham had the strength to sacrifice a child if need be, he did not have the ability to produce one. For the first time Abraham had to sit back and leave it to G-d, trusting that G-d would do it without any help. This new level of faith that Abraham displayed was considered by G-d as righteousness.
In Pirkei Avos it is written: "In a place where there are no men, become a man!" The Vilna Gaon explains this as follows: In the Torah, the word "ish" is used in two places, with different meanings: (1) "The L-rd is a warrior ["ish milchama"], the L-rd is his name" (Exod. 15:3); (2) "And the man ["veha-ish"] Moses was very meek, above all the men on the face of the earth" (Num. 12:3). At first glance the "ish" in these two verses seem to have opposite connotations; to be a warrior one cannot be meek. The first use of "ish" is connected with the trait of heroism, "gevura", and the second with that of modesty, or taking one's own strength out of the picture. To this the Torah sets a new level of greatness, asking us to integrate these two traits, and become a real "Ish"!