Depends on what you normally use it for.
List some games you play and the rest of your setup to see if underclocking it will affect it in a bad way.
Look for a program called RivaTuner and use that to under/overclock. When using rivatuner make sure you tread carefully as you can mess things up.
You could also tell us what your idling (at normal when not playing games) temperatures are and your load (when playing games and stuffs) temperatures are, this way we could use RivaTuner to lower the fan speed and not be required to underclock it.