You can do any degree you like. From what I've been told, the interview is more of your qualities as a person, and how you'd do as a doctor, rather than just academic ablities (thats why the have GAMSAT and use your Uni marks). I spoke to a lecturer at Sydney, and she'd done Music up in Lismore (southern Cross, I think) - very nice lady, not what i expected of an academic at USYD) and she got in. She said do something that really interests you if you know that you definatly what med in the end, because that way you'll get good marks. GAMSAT you need the equivilent to HSC Chem. The Interview is assessing your suitability to Medicine -- you can have the marks but be a social recluse and have no idea how to deal with people, and thus wouldn't make a great doctor.
I'm planing on doing Grad Med too, but an doing Nursing at my under grad degree because even if I don't get into Medicine, I can still work in the health field and have the ability to help people. That and, working at a rural hospital, RNs do the job of interns anyway...
I'm hoping for Sydney too, because one of their rural hospitals is Dubbo Base - and, being in the Macquarie Area Health Region, means I can go out to Coonabarabran District too. I'm told the fact I really want to work in the bush is a big plus in the interviews, and the fact I know about the region I want to work in shows I'm serious about it (ie, not just in it for the scholarships). Working for the Royal Flying Doctors Service, however, would be absolutley amazing!