Well, I'm not really sure about the Australian exchange rate, but given that it's a continent a bit out of the ways, I wouldn't be suprised if America winds up being the cheapest destination for you, if you'd like to make your money stretch.
There is an awful lot to do here, and an awful lot to see, but it is so varied that you really might be well served going to the library and finding a picture book of American cities/regions. Some may suit your taste more than others. We have everything from rainforests to deserts to everglades (read: nice looking swamps), to mountains, to hills, to steppes, to, well, you get the idea. Everything. But, most of the time, it is a very long hike from one typography to the next.
OTOH, if you have three weeks to do it, you could conceivably spend one in different regions and get a grand taste of our nation. If I had to pick your three sites, I'd say:
1. NYC (It's like Paris, no matter how much you might hate the place, everyone's got to see it once);
2. A southern state (I'm partial to North Carolina in the Charlotte region, myself - the people are friendly, the music and food are great, the views are spectacular, and the temperature is reasonable). Charleston, South Carolina, is also a nice place to visit (you can see the Yorktown there, if you'd like to make a historic stop).
3. Somewhere out west - like Las Vegas, or California.
The nice thing about this set up is that you could probably swing it on your budget, assuming you can get some good flight deals. NYC is very pricey, but the south is certainly not, and Las Vegas has to be the cheapest place in the world to visit (well, to get, and stay there - not necessarily to play
)
Hope this helps,
