I always have received my preorder on the actual release day, the 21st in this instance. They charge either right away or at time your order is processed, which has varied for me in the past. I'd have the $$$ in your account when you place the preorder.
I've preordered the 4 and 4S. Both times they did a "pre-charge" to my CC account. I guess it's to make sure you're good for the amount. That said, I'd have the money ready in whatever account you plan to use.