I own the same card (7900GT from EVGA). Mine idles at 47c in my Antec P180B. I assume your card temps are in C, and your room temp is in F. 100C is high for a video card at load. How is your airflow in your case? I noticed that you have your CPU overclocked. I am in a very similar situation. I have the Asus P5B-D with E6400 (@3.4GHz), my P180B only has two case fans on low. At load my 7900 only reaches 64c. The link I provided is a HDR Stress Test that does a great job at loading your GPU. I would suggest you run that, and watch your temps through nvidia's monitor.
Something doesnt seem right that you are getting idle temps around normal and load temps sky high. If you have poor circulation with high mobo temps, I could understand GPU temps in the 70s-80s, but not over 100c. Have you fiddled with the GPU cooler at all, or tried to OC it?
If the temps are still a problem you could buy a Thermalright HR-03 to cool your card. I just ordered one. I cant stand the noise from my GPU fan, its the loudest thing in my case, and I cant wait to get rid of the thing.
Is your whole system running hot, or just your video card? What are the rest of your temps?
Heres the HDR Stress Test:
rthdribl - Real-Time High Dynamic Range Image-Based Lighting 1.2 -