What events recorded in Scripture must be historical for you to affirm the truthfulness of Christianity?
As someone who was once a very devout Christian (Catholic, if anyone asks), now an apostate, I can tell you that this is a very important question, but perhaps no longer relevant. One must ask if the first Christians who wrote about the Gospels accepted it all as historical–certainly Christians like John Dominic Crossan don’t even need the resurrection–as it is recounted in the Gospel narratives–to be historical in order for him to accept Christ. At the same time, others like NT Wright (or these guys) have to accept even the most outrageous positions on historicity–like the dead rising from the graves and walking all over Jerusalem from Matthew’s Gospel (27.52-53).
The difficulty in this question is in deciding, for yourself, which is more important: the historical truth or the theological truth? I am certain that early Christian minimalists didn’t care for the historical reality of the Gospels–if they did, there would not four canonical ones (and there certainly would be dozens of noncanonical ones!). The theological message above all else seems to have been more valuable a truth and thus why we have multiple theological messages in the narratives (even between the epistles and pastorals). The historical value of the text was only useful when it suited the functions of the theology.
For example, Paul believed that the resurrection of Jesus was a historical, functional event (whether on this earth or another, is still up for debate in my opinion), but then again Paul does not once quote Jesus on anything nor does he cite any specific examples of his life to make a point (except for matters of theological significance like the crucifixion and resurrection). It is not until the church fathers (mainly from late antiquity and Latin Christendom) that we find the historical value of the narratives taking a precedence towards explaining theological values.
However this is a slowly dying trend; I believe with the continued advancement of science many will search the Bible for that theological meaning as the historical value continues to diminish. Still, I would become a Christian immediately if the resurrection of the figure of Jesus was proved to be a historical event. Likewise, if the resurrection of Ishtar were proved to be a historical event, I would start singing her praises.