Unfortunately, I don't have an iMac G5 with a built-in iSight at home. I did a search here in the Front Row forum looking for a report from someone else with one of those Macs who needed to re-install Front Row, but I didn't find one.
If you had an Intel-based Mac, you can use the "Install Bundled Software Only" option on the original Mac OS X install disk to re-install Front Row. But, from previous topics I've read here, Front Row does not appear in the "Customize" screen for the G5 iMacs.
If I had to guess, there'd be an option in the Mac OS X installer itself. But since I don't have one of those Macs, I couldn't test it myself to find the screen or check box. We do have one at work, but I'm not sure I could find the install disks for it and test it there. (I'd have to interrupt someone at work too!) We install all our Macs from a FireWire hard drive that has a standard bundle of software and Mac OS X. So, the install disks themselves are not usually important. I may have time on Monday to do a little search for it if no one else responds here.
-Doug