The older Nvidia's could hit 100 and be fine.
I would definitely keep the 7900GT below 60, but it would take at least 90 to permenently fry it I'd imagine. For the sake of your computer as a whole though, keep it below 60. Also, the 7900GT's would really OC that much due to a lag of voltage, so doing a volt mod would really allow it to fly as it's OCing capabilities are great, it's just limited by it's voltage.