I took that scene as being sort of a parallel to the line in the sand that William Travis drew during the last days at the Alamo. Basically, Buffy, like Travis, was offering her troops a choice as to whether or not they would make the commitment to fight, not to become Slayers. Realistically, Buffy, one vampire, and a dozen or so inexperienced girls, Slayers or not, really shouldn't have been able to accomplish what they did. It was a suicide mission, and they all knew it from the beginning. That was the choice that they were called upon to make.
On that same note, I think when Buffy told Spike, "I just realized something. We're gonna win," she wasn't necessarily speaking about herself and her little band of fighters. Even if they had every one died there in the hellmouth, if the First Evil had escaped its confines and ran rampant throughout the world, it and its minions would face literally thousands of Slayers ready to continue the fight. (Don't ask me how she thought that all of those girls would somehow organize and coordinate the fight in time to be effective. She probably just figured that Joss would figure out a way if there ever was an 8th season)