If you have the Radeon 9800 video card, it may be the fan on the video card that is causing the noise, not the G5. There were quite a few of these cards sold with loud fans. I got one and it drove me crazy. In the end I bought a quieter fan and installed it.
Otherwise if you think it is the CPU fans you can go it to the power saving preferences and there is a "performance" option in there which you can set to automatic, higest or reduced. If you set it to reduced the fans go much quieter, but it does it by slowing down the CPU so there is a tradeoff there obvioiusly.
Also make sure you have the latest 10.3.4 OS. I can't remember whether it was 10.3.3 or 10.3.2, but in one of the Panther updates they did something which greatly improved fan behavior - made the speed changes much more even and the whole thing quieter overall.