SAT Subject Tests

Couple questions:

  1. How many subject tests should I send in to colleges? I've heard the popular amount is two, but does sending in more than two benefit you?
  2. How high should scores be for students aiming for Ivies? I currently have 790 in Math and 760 in Chem. I'm retaking the Math to get an 800 and probably not sending in Chem as it is too low. So, what's a good threshold for Ivies?