This is definitely abnormal. I have nearly the same machine you do, except that mine is the 2.3 GHz version. It runs pretty quietly. I have heard the fans take off only once, and that was when I was trying to boot an Ubuntu Linux Live CD on the machine (Ubuntu didn't support the thermal sensors and fan control mechanism, hence they just ran full throttle - gave up on the idea of dual booting Linux on my G5 after that pretty much permanently!). I agree that when the fans take off, they are LOUD.... drive you out of the room loud.
A healthy G5 shouldn't do this. Could you be having kernel panics? Presumably you would know this - I understand you are greeted with the Mac equivalent of the old Windows "blue screen of death" when this happens.
Do you still have your Tiger install disks? If you can boot off of those and the fans behave in the normal nice-and-quiet mode, then you are lucky (?) and just your software is corrupted. Else, your hardware needs help.
If your software is corrupted, you can reload the OS. If your hardware needs help, you will have to take it in or send it in for repair.
