Depends on the processor as each has its own thermal design. The general rule is to never exceed 60 degrees on a C2D/C2Q under prime 95. I know some people are willing to push it up to 70, but my standards have always been more conservative.
When gaming, your temp should generally not surpass 50 - 55 degrees.
True, it can only be moved around until it's molecular state changes, the molecules still exist in some form though but for the purposes of moving the heat away from a processor, heatsinks and fans or cooling blocks on a W/c system are what we have to work with