There is something wrong with either your GPU temp sensor or you hvae no airflow in your case. Your GPU should not be past 85-88 C with 100% GPU fan running. If you keep hitting 100C, your GPU will soon die.
That's an old GPU. If you know how to remoce the casing (very simple) and blow out and clean the fan, it will also help. If you have a friend who would like to buy it at a fair price (about $25 - $35), I would sell it and get a bew 9800 GT for around $85.
At the moment i am happy with my 8600 GT. I've just bought it temporarily until christmas when i can buy a better card. I've just put a case fan at the back of my PC blowing air out and it has lowered my idle temp from 70 degrees to 57! Now for one at the front!