I updated to Mavericks — now my G5 is so slow I can't stand it!
Mavericks System Requirements
Mavericks requires an Mac with an Intel processor (system requirements), so I don't know how you installed it on a G5. Can you provide more information on the model of Mac you have, including the year?
Simply Overview for Reverting to an Previous OS
There are two ways to revert to a previous OS version, both require erasing the computer and starting over. The difficulty and time required will depend on the status of your system backup(s), or for that matter if your even have a backup. Below is a simple overview of the steps required. Further assistance can be provide once we determine your situation.
Made Backup Before Upgrade
Hopefully you created a backup before the upgrade. If so, you should make a separate second backup of any files/documents that you have worked on since the upgrade (if possible, make a second complete backup). Reinstall from your pre-upgrade backup and then copy over any files/documents from your second post-upgrade backup.
Did not Backup Before Upgrade
If you didn't make a backup before the upgrade, you'll need to make one now. Once you have the backup complete and tested that you can boot from it, you can reinstall the pervious version of the OS that you were using. Once the system is backup and running, you can use Migration Assistance to move over all your Apps and documents from your post-upgrade backup.