Bill's right on. If you already have a VHF ant on your mast, you have most everything you need. You'll get more bang out of adding hieght to your antenna over most any other improvement. If your VHF isn't high on the mast, putting one up there is still reasonably cheap.
It sounds like there's a little confusion on digital radio. In the US, there is what is called HD radio which is free and broadcast on the regular AM &FM frequency bands. You do need a compatable receiver or converter though. XM is a satellite service that operates at a different frequency using satellite and some ground based transmitters. You need a different receiver for it. Satellite radio is a paid for service that would improve your choices since there are hundreds of channels including talk, news, & every music genre ever thought of. I had it in my pickup for a while and there was a 24hr Elvis only channel, loved it. It cost $14 a month and it wasn't worth it to me, but could be for others. Don't know what's available in Austrailia satellite-wise, so I would recommend the same device Bill did. If something like HD radio becomes available there, and I suspect it probably already is to some degree, you would still use the same antenna and performance would be more reliant on a good antenna system.
Regular AM & FM, to some degree, are affected by static interference that sort of rides the signal. You hear static or scratchy noise but you also can hear the original transmission much of the time. Digital radio will lose packets of data and just go silent so portions of the audio will be muted. The plus side is the method used to send the digital audio is less prone to being interfered with by outside static noise.