Temps will vary alot depending upon the load and particularly the ambient temps of the room. 38°C is a good idle temperature, and as a rule, you want to try to keep it below 60°C.
Just be aware however, that with Dual core CPUs, the bottom Core will be between 0.5 and 3°C hotter (further away from the HSF).
Regarding thresholds, I am unsure. As far as I know though, the Intel CPUs use throttling to control temps (i.e. if it is getting too hot, it will automatically downclock itself), and will often just switch off if it overheats...don't quote me on that though!