How much does your act score actually matter for getting accepted to northwestern or any other top university?

If your ACT score stinks, they're not going to let you in. However, that being said, top universities reject lots of applicants with great test scores every year. The keys to getting accepted to a top-tier school (other than great test scores and grades) is a record of extracurricular involvement, leadership, volunteerism, and things that set you apart. Of course, these don't guarantee admission at top-tier schools, but they help you have a better chance.