As someone said, many places don't let first years overload- so i would check that before you try to.
I know people that overload by one subject- but I have rarely heard of anyone overloading by two subjects (ie. a whole 12 credit points). I think 5 is achievable, if you put the work in, and don't have heaps of other commitments, however I think 6 is taking it a bit too far. I think it would be very difficult to go well overloading by that much.
The thing to remember is, whilst you may have calculated the actual contact hours that is required by doing more units, you have to think about the amount of outside reading and work on assignments that will need to be completed.
- What is the assessment schedule of the units- do they all have end of semester exams? (this would make it a pretty hectic exam schedule) or do they require lots of essays during the semester? Will you end up having a heap of essays/assignments all due at the same time?
- What is the reading load of the units?
etc.