i'd say 75c on a constant full load is too hot
and no.. there is no formula for voltage compared to clock.. each chip is different.
i wish we had a guide for you.. for i7 is just too new.. but me along with a few other would be more than happy to help you out
stock heatsink can probably get you 3.0ghz for everyday use for the most part.. even 3.2... i'm still on stock heatsink until i get this thing running on my watercooling loop...
and yes, buying a 40 dollar aftermarket heatsink would make a tremendous difference.. this stock one is absolutely garbage
i hate it