There is no fixed temperature above which CPUs become unstable. It all depends on the individual chip itself. CPU temps up to about 75-80C are well within tolerances as a general rule.
An iMac consistently running at 68C when only using Safari and not hitting any sites using CPU intensive things like flash and/or applets might be a little warm. However, it all depends on the room temperature and your actual system load.
It is probably worth checking that your iMac is getting sufficient ventilation. That is, nothing blocking the intakes at the bottom, or the outlet slot at the rear, top of the iMac. If you have the heat on or other heat producing things near the iMac, that the hot air is not blowing onto the iMac. The iMac is not in an enclosed space such as a wall unit or tall shelf/desk construction. Etc.