Do not run the processor over 60C for long periods of time. Intel says "not good."
For voltage increases, most E6600's seem to hit a wall at 3.2 w/ stock voltage. I bump mine up in quarters of a tenth at a time. Like from 1.3-->1.325, for example, and test it to see if it's stable. If it isn't, then I'll bump it to 1.35. And so on...
In my case, my 6600 will get to 3.2 easy on stock voltage. But I have to bump it to 1.4 to get it to 3.35, and I have to bump it to 1.5v to get it to 3.5, and then 1.6 to get to 3.6. And then 1.7 (
) to get to 3.8ghz.
It all comes down to luck of the draw. Some overclock better than others. That's the joy of this game. Unpredictability.