Hi Sherrod!
Can you tell us which vendor and model optical drive was originally installed, and which vendor/model optical drive you replaced it with?
Certain drives have limited support with different versions of OS X. When Apple releases new OS updates or upgrades, they usually include additional optical drive support.
#1 Try resetting the NVRAM/PRAM:
http://docs.info.apple.com/article.html?artnum=2238
#2 Use Disk Utility (from the Utilities folder) to repair disk permissions, then reboot and try again.
#3 If the above 2 steps fail to resolve the issue, update to Mac OS X 10.3.9 (the last update to Panther):
http://www.apple.com/support/downloads/macosxcombinedupdate1039.html
#4 Make sure your AGP G4 is running with the latest firmware:
http://docs.info.apple.com/article.html?artnum=86117 (I think it probably is since you're running OS X...but still double-check)
#5 If all else fails, try using PatchBurn 3.1.4 to create a profile for your optical drive:
http://www.patchburn.de/download.html
#6 Check out this compatibility database to ensure the optical drive you installed has been tested to be fully compatible with your system (note that it might not be listed):
http://forums.xlr8yourmac.com/drivedb/search.drivedb.lasso
If nothing else...
Open the Console application (in the Utilities folder), which will open the console.log file be default. From the File menu, select "Open System Log."
With both Console windows open, insert a DVD-ROM disc and see what messages (if any) are added to the bottom of these windows. Copy them and post back to us, along with more info on your optical drives.
Here's some other misc. articles I dug up on Apple's site:
http://docs.info.apple.com/article.html?artnum=58382
http://docs.info.apple.com/article.html?artnum=58707
Keep us posted!