When syncing new iOS6 devices to your mac, you need to be running Snow Leopard (10.6.8) at LEAST in order to update your iTunes to 10.7, which is what iOS6 needs to communicate. If you're running 10.5.8 and need to upgrade, please call Applecare. The disks are only available over the phone through them for $19.99 (single-user license).
So Choccs - you need to have your mac upgraded to Snow Leopard at least before you can sync your new phone to it. Before you order it, make sure that your computer can run it. You can find the tech specs for it @ http://support.apple.com/kb/SP575.
It's can be a pain, but the firmware from Leopard to iOS6 is just not compatible. Also remember if you need help after getting Snow Leopard that you do receive 90 days of complimentary phone support for upgrading the computer. Good luck!