If you have more than a passing familiarity with Macs then consider trying this on your own. Beware that I did not do this on my Tiger Mac yet and there may be something I missed, so proceed with caution. If this breaks your eMac you will have to restore it on your own.
Create an Administrator account on the eMac and install iLife in the usual manner.
Then, start the Mac in single user mode and do the following
Mount the filesystem
/sbin/mount -uw /
Load Directory Services (the following is all on one line - no line breaks)
launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
Delete the user account username you used to install iLife
dscl . delete /users/username
Delete the file .AppleSetupDone to indicate the Mac should start up with the "Welcome" animation and new user setup instructions
rm /var/db/.AppleSetupDone
Shut it down
shutdown -h now
This will result in the "new, unconfigured Mac" you seek. Verify that it does - create a new user and ensure the iLife apps you installed work as intended. If you are satisfied with the result, then repeat the above to delete the user you just created, and don't restart the eMac.
I would also be remiss if I did not point out that you are obligated under Apple's license agreements to provide the iLife and OS X discs along with the eMac, and erase any other installations you made with them on your computers.