My HIS 4830 has been going up into the 100+C temp range while playing games such as CoD4/5 and some others. Even though my card hits over 100C it doesn't slow down, crash or nothing. My card seems to idle normally at 33Ccore 38Cvram but once I turn on CoD4 or something it heats up to 100+ almost instantly. I was wondering if the HSF could be not seated right on the card.
Use catalyst control center, it will allow you to manually control the speed of the fan in the ATI Overdrive section. You also want to boost the airflow in your case, even the worst cooler should hold the chip at safe temperatures for a few minutes. Run furmark and see if your heat spikes, also make sure to watch the case temp, if that starts rising rapidly then its a problem with case airflow not card airflow.