Is my GPA good enough for top-tier graduate schools?
The 3.6 looks a bit weak, but there's lots of other relevant information that's not in your question.  For example, a poor grade in first-year calculus wouldn't matter so much if you have solid A's in more advanced courses; on the other hand, solid A's in calculus followed by lower grades in more advanced and theoretical classes would be a serious problem.  Also, letters of recommendation are very important in graduate admissions.  If you've done a research project and your supervisor writes an ecstatic letter about the quality of your work, that can compensate for lower grades in some classes.