form api: extend "save configuration" and "reset to defaults"

I'm using Drupal 4.7 Form API and want to add some extra code to "Save configuration"
and "Reset to defaults" on settings page? Is it possible to extend the default functions?
What I want for "reset" is a call to DB that removes all according variables instead of
only reset the forms on settings page. And on "save" I want to modify some variables
depending on the user input.