Contrary to popular belief, legacy does not offer a lot of help to applicants. Legacy only applies to applicants who are on the border. (ie. Two students who have identical stats, 3.5 GPA, same SAT, same HS everything. Whoever has legacy will get in.) Legacy will in no way help a 3.2GPA student get in when the average admittance has been 3.7GPA.
If anyone has researched the facts, only 15% of the admitted students had some type of legacy.
USC is a private school, so there is no straight formula for getting admitted. Someone can have amazing GPA, but can still be rejected for whatever reason. (ie. Didn't do any requirement courses, choosing a competitive major, having a weak essay, etc.) I can tell you right now, I know a bunch of 3.8+ GPA students who have been rejected and several 3.4GPA+ who have been admitted.
Remember, when you are calculating GPA, you can only calculate classes that can transfer over to USC (mostly classes that apply to USC GE or USC preq). Just because your transcript GPA is a 3.8, it makes you a 3.8gpa student. You could have A's in PE, intermediate alegra, dance, and other classes that USC doesn't take seriously that pulled your 3.5 to a 3.8gpa.
http://www.usc.edu/dept/ARR/articulation/
The only classes USC counts are the one listed in your specific school articulation agreement and your specific major's prereqs.