It also depends on which American university you're applying to. Some schools have their own GPA system - the University of California schools, for instance.
Thankfully, American schools consider extracurricular activities (sports, clubs, community service, organizations), your high school GPA (sophomore, junior, and senior year), SAT 2 Subject Tests (the number of these depends on which college you apply to), and either the SAT or the ACT (and sometimes both)...
If you want to look at some colleges, just go to
www.collegeboard.com.