Since the form submission handler has access to the $form
variable, you just use the following code.
$foldername = $form['select_list']['#options'][$form_state['values']['select_list']];
To make it clear with an example: $form['select_list']['#options']
is an array like array( 1 => 'first folder', 2 => 'second folder')
, and $form_state['values']['select_list']
is a value between 1 and 2.