More work on the ISS is very important in developing ways for large(ish) amounts people to survive in space for extended periods of time.  
To make our way to Mars, it's estimated that its going to take about a year each way with current technology. Even if we do manage to greatly improve matters, I'll be betting it'll be a very long trip, on return. We can't have potential Mars travellers in the same conditions as the Apollo 11 crew.

I think the space elevator will also be important, as it will provide a much cheaper way of getting thing to and from the Earth. Setting up a space elevator system would probably have to come before terraforming the Moon, as this will involve transporting large amouts into space often.

Post edited at 9:46 pm on Feb. 3, 2007 by flatplat