Which OS version? You are posting in the OSX 10.5 forum but your profile says you are using 10.6...
If you want to upgrade from 10.5 to 10.6 you will need to buy the Snow Leopard DVD.
Upgrading from 10.6 involves a system download online. Why not just install 10.6, update to 10.6.8 and then do an online download from there to make things simple?