Max safe per core temps are 98ºC. The CPU will start auto-throttling in the low 90sºC to keep from damage by heat -- I have verified this by deliberately overclocking and over-volting too far.
To get accurate temps, you should use a program like Core Temp that gives you the temps for each individual core.
As far as "comfortable" temps go, that's a personal choice. I limit myself to 75ºC peak temps at full IntelBurnTest load (using Maximum setting) on my Sandy Bridge processor.
A good guide
for ASUS P8P67 overclocking.
For benchmarking and stability testing I use FurMark, 3DMark11, Cinebench 11.5, MSI Kombustor (packaged with MSI Afterburner graphics card overclocking utility), HD Tune, PassMark PerformanceTest, and SiSoft Sandra.