If you're looking for magic number you won't find one. According to Intel spec your CPU voltage will be safe up to 3.75v and I wouldn't exceed core temps of 80deg C on stress testing such as Prime95 (blend) or LinX (full memory usage). But for the clock itself, the sky is the limit. Keep in mind exceeding Intel's specs voids your warranty and can shorten your CPU/other components life time.
1.375. Missed the decimal point.
You load the processor using prime 95 small fft test at stock, use Realtemp to monotor the coretemps. If below72C, begin the overclock. Don't let it load above 75C long. Normal usage is below the load test temps.
Overclocking with a stock CPU cooler isn't a good idea.