I am using a Telstra/Bigpond Next G PCMCIA card,  it gives you unlimited time and max 1Gig  of traffic,  current cost is $85.00 per month.  I would recommend  the PCMCIA card over the USB as well and I would highly recommend a external antenna .   

I have achieved consistent 100Km range from land from Perth to Melbourne,

I have basically never been out of rang yet except when we went across the bight and in some areas on the west coast of Tasi, best range that I have had was 180Km.

The Next G system seems to have the same range as the old CDMA but is a lot faster

 

There are other data plans with Telstra/Bigpond that you can use and these vary in cost, the above one suits us.  (Note that you can change the plan once a month to a new level at no charge)

Feel free to email mail me if you have any questions