I answered below, but then I realized you may want to mod the card physically, if so sorry, IDK.
I think you are stuck at max 1.263v without hardware mod. Safe temp is something absurd, like 95*I think.
I use Ati Tray Tools to make hardware profiles for my over- and under-clocks. ATT lets you pick between 4 different voltages that correspond to the power saving tech in the cards. The hardware profiles let you change the fan settings as well.
My 4870 is running at 800/1100 most of the time, at 1.263v. I've tweaked the fan settings to run at different speeds for different temps, which is nice. It never gets over 70* ever for me, fan never goes over 65%. Yes its loud, so I turn my headphones up