hi göran, if it's not leaking - let it be. there are other possible ways to get control over the heat issues. did you try a different graphics card from one of your other powermacs (as i mentioned [http://www.ifixit.com/Answers/View/15349/Fan+speeds+up+by+-+mouse+or+graphic+card+-+G5+2x2%2C5GHz|here] ??
maybe you should try a non apple fan system to support the graphics card - use a free slot and put a fan in there - or use a complete fan system for your card, or a few ultra silent fans to cool the card down. many possible ways to get the temperature down ;-)