I think you will find that the grey discs are machine specific. You having bought them, I hope I am wrong or at least can return them. Let us know how you get on.
Type in 'Are 10.5 disks machine specific?' into the apple search communities box above. The answer will be there.
Message was edited by: seventy one
You need a retail Leopard install DVD that looks like this
Mac OS X 10.5 Leopard installation system requirements
Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).
If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:
MB427Z/A Leopard 10.5.1 install DVD
MB576Z/A Leopard 10.5.4 install DVD
MB021Z/A Leopard 10.5.6 install DVD (single user)
MB022Z/A Leopard 10.5.6 install DVD (5-user family pack)
Installing Mac OS X 10.5 Leopard
Mac OS X 10.5 Leopard Installation and Setup Guide
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.