How to show extreme gratitude in an email?
Politeness never hurts. "Thank you for your very detailed and positive information" could be one option or something along those lines. I am sure most people providing this type of response do not expect much in return (not that they do not deserve it). But as with many other instances, keeping the thanks short and concise is necessary, do not overwork it because that just seems suspicious. Remember that the person is doing their work (well) and should be praised for just that. 

If the school to which you are applying is large then it is not very likely you will be remembered and so the response will have little significance other than to show appreciation. In a smaller school, however, your politeness may be noticed and can help you build good relations with administration for the future so politeness never hurts.