I have taken the ACT, should I take the SAT also?

I already took the ACT, and am very pleased with my score of 33. This was without the writing section. I am thinking about taking the SAT, but if I don't do well on the SAT then colleges can still see it. With the ACT I have to send out my scores for them to see it. From the PSAT, I can expect my scores to be around 630 for Reading, 690 for Math, and 610 for Writing (I think PSAT scores can translate like that). If it helps, some of the colleges I am considering are Stanford, Carnegie Mellon, and MIT. Is it worth it to take the SAT? If I don't do as well on the SAT, can I still ride on my ACT score?