It depends on the video card. But for the most part anything over 100C is usually a bad sign, however, some run MUCH hotter than others. I've had video cards that hit 100C and it is considered normal. I believe 120C is usually the "melting point" (not literal) of most cards.
Alot of people will tell you something different, but a video card with a smaller cooling system will run hotter than the same card with a much better one. It doesn't mean it won't run effectively, it just means it is running hotter and leaves less room for overclocking and can potentially lower the life of the card.
^ I think most cards that I have owned have topped 80C on load with the stock coolers. 80C is pretty normal under load. My old x1900xtx used to run at 100C on load at times. I OCed with a thermalright cooler which got it down to 75C. But I used a 7950GX2 and the one board used to run mid 90C.