Yeah... 120C is really pushing the limits. Seriously shortening the lifespan of a card. But, at the rate that software/games are going, they shorten the lifespan faster.
Anyway, there was a article, i forget where, but they were saying that ATI gave a few review sites a special tool that they developed for OCin' the X1800 & X1900 cards. The site was able to reach 780/1800 on the stock HS & FAN with another 80mm fan blowing directly over the card. No artifacts.
I'm sure with the right tool and liquid cooling the R580 can go much higher then that.
So MOLE, don't worry. Go into CCC and run the test app in overdrive. It will then tell you that you can run your card at 687/799, crank it up and run 3Dmark05. You'll notice about 500 more points easily. The temp change will be about 3C or 4C.
Crank her up dude... no worries.