Hang on a minute
One thing to bear in mind is that Leopard is nearing end-of-life, and there are plenty of places you can buy it for less than list price. Try Amazon, look at ads towards the back of MacWorld, and otherwise hunt around, and I bet you can come up with a significantly cheaper price. You can probably find Tiger even cheaper, but if you ever hope to upgrade to Snow Leopard it would probably make more sense to buy Leopard as cheaply as possible, because then the upgrade would be much less. B. t. w., I have no idea how Apple is going to establish that a customer already owns Leopard. So I think I'd steer clear of buying a used copy of Leopard since they might require some kind of proof of purchase documentation, and they might not honor their discount deal for second owners.