Great question -- you might get a variety of different answers here, since the answer may be different for each school that you are applying to. Based on my experience applying to college, speaking with admissions officers, and being involved with application review, SAT scores are just one of many factors that are considered. Maintaining good grades will definitely help you to improve your chances. That said, the direct answer to your question is that the SAT score is important for admissions, and can affect your ability to gain access to a top-tier school unless other elements of your application are strong.
If you have already taken the SAT and you are unsatisfied with your score, you may be able to study and take the test again. Many people take the test twice. However, it may be a good idea to do this only if you believe that you can significantly increase your score (for example if you score higher on sample tests than you did on your SAT).
If you do not believe you can improve your SAT scores (for example if you do not have enough time before submitting your applications), then you should focus on making sure that the other aspects of your college applications are very strong (working on your essays, your short-answer questions, your grades, and your extracurricular activities).
Don't lose heart - my SAT score wasn't as high as I wanted it to be, so I took it a second time and worked hard on the other elements of my application, and I went to a good college.