Actually I think getting Austrailia is most important for a couple reasons.

For starters there is only one entrance to it so it is easy to defend.

It is kind of out of the way of everything else so everybody else is killing eachother while you are building up your army around your base.

Also it is very close to Asia. So there is a good chance that if you take control of Austrailia you will eventually take control of Asia as well. And having control of those two continents gives you about 14 men every turn.

Also though I do not think Risk online would work very well. You would not find very many people willing to sit through up to a two hour long game. Especially if they were not doing very well in the game. It would be very frustrating trying to play online because 3/4 of the players would quit half way through the game.