While some claim to have had success with things like repairing permissions and lens cleaners, typically this is a hardware issue with the drive.
As for how easy it is or isn't to do... I haven't worked on that many G5 iMacs, but in large part it depends on what G5 model you have. If it's any with the iSight webcam, that instantly makes it many many more times a PITA to get the unit open. If it's an older model, then it's a relatively simple matter, and you should be able to find guides on the ifixit site.
If it is an iSight model, take it somewhere, otherwise you're going to bend the clips in the bezel, and the thing won't sit right when you put it back together. Some other place bends the clips up, they're on the hook to replace the bezel on their own dime. And you won't be able to take a PPC unit to an Apple store, they'll just tell you it's too old to work on.