I'm studying in the US for the next few years, so I had to get a friend over there to help me with it all.
The GPA system in America is rather complicated. GPA's can actually go up to 4.5, as students can choose to undertake AP (Advanced Placement) and College-level courses which allow them to gain 4+ GPA scores.
So, if you completed Extension courses this year, there is a good chance your GPA score will be higher. Also, different courses contribute different ratios of points to your average. For example, their standard English marks only contributes half of what Level 2 Mathematics marks do.
I don't know if I explained that very well, but oh well. Try looking it up on wikipedia.
Generally, if you apply to a university in the US as a foreign student, there will be a department that assists you in converting scores. It's also a good idea to sit your SAT's before you complete your application. Don't worry, they are so much easier than the HSC.