To mirror or not, and how

I am redeveloping a large international web site that may or may not need to have mirrors on multiple continents (for performance speed). Has anyone ever seen a good discussion of how best to decide this? The second part of the question is what's a reasonable method to use to seamlessly send visitors to the right mirror (without their needing to decide anything) while keeping their browser's address field still showing what they expect? Brownie points to anyone who can answer both!
-David Booker