Becoming a psychologist takes a long time and a lot of effort, in my opinion.
Your best chance would be a B. Psychology (e.g. USYD) or B. Arts/B. Psychology (new course USYD). The UAI for these courses are very high, 98ish? You do 4 years undergraduate. If you're doing B.Psych you need to maintain a distinction average because you have to do honours. Then theres another process after you graduate before you can practice.
But if you had to choose between B. Arts and B. Arts/B. Science, definately go for B.art/b.science. As far as I know you cannot meet the undergraduate requirements through B. Arts to go on to become a psychologist...You can major in it but you need to also major in an arts subject (because psychology is a science), which doesn't leave much room for psych.
I'm not too sure about all this myself. You should look into it, go to usyd's website and look up the B. Psych course.