Yes it is the right Disc...
2Z691-6040-A (OEM) Drop-in DVD...
System Requirements
Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor. User must already have 10.4 or above to install this 10.5 upgrade.
Compatibility
eMac (2005), iBook G4 (Mid 2005), iMac G5 17-inch (ALS), iMac G5 20-inch (ALS), iMac G5 (17-inch iSight), iMac G5 (20-inch iSight), iMac (Early 2006 17-inch), iMac (Early 2006 20-inch), iMac (Mid 2006 17-inch), iMac (17-inch Late 2006 CD), iMac (17-inch Late 2006), iMac (20-inch Late 2006), iMac (24-inch Late 2006), iMac (20-inch Mid 2007), iMac (24-inch Mid 2007), Mac mini, Mac mini (Late 2005), Mac mini (Early 2006), Mac mini (Late 2006), Mac mini (Mid 2007), Mac Pro, Mac Pro (8-core), MacBook (13-inch), MacBook (13-inch Late 2006), MacBook (13-inch Mid 2007), MacBook (13-inch Late 2007), MacBook (13-inch Early 2008), MacBook Air, MacBook Pro, MacBook Pro (17-inch), MacBook Pro (15-inch Glossy), MacBook Pro (15-inch Core 2 Duo), MacBook Pro (17-inch Core 2 Duo), MacBook Pro (15-inch 2.4/2.2GHz), MacBook Pro (17-inch 2.4GHz), MacBook Pro (Early 2008), Power Mac G5 (Early 2005), Power Mac G5 (Late 2005), PowerBook G4 (15-inch Double-Layer SD), PowerBook G4 (17-inch Double-Layer SD), Xserve (Late 2006)
http://www.welovemacs.com/2z6916040a.html
I'd try to find another Mac to make a DMG of it & burn a copy to see if that works better.