I would recommend building your own PC if you have the knowhow. Package deals are often a waste.
And here's why: See the processor on your "more expensive" one?
http://www.cpubenchmark.net/cpu.php?cpu=AMD+FX-4200+Quad-Core&id=2118
I'd go for at least an AMD FX-6300 if you wanted something cheap; you can find them for under AU$150 if you look properly and they have a better benchmark and a high value rating (CPUMark Score / Cost):
http://www.cpubenchmark.net/cpu.php?cpu=AMD+FX-6300+Six-Core
For graphics you can pick up an nVidia GeForce 650Ti 2GB OC edition for about the same price, which would bring you up to roughly AU$300.
For a case and PSU, fact of the matter is you won't be looking at the case half the time so just go for the cheaper ones. You could get a decent case and PSU for about AU$50. So far the total is $350.
Next, you need a motherboard. You need to make sure the CPU fits the socket on the motherboard. The AMD FX-6300 for example is an AM3+ CPU, and you can get an AM3+ motherboard for between $50-100, so on average AU$75, which will get you USB 3, 1xPCI-Express, a couple of PCI slots, 2xDDR3 slots, HDMI out etc. Brings the total to about AU$425
Next, RAM. I'd go with 4GB DDR3 1600MHz for starters and if you really need it upgrade to 8GB from there. Chances are 4GB will do just fine for a budget gaming PC, any more would just be over-speccing IMO. That's another AU$45 or so. Total AU$470 so far.
Add wireless card: AU$30 - Total: $500
Add DVD Drive: AU$30 - Total: $530
Edit: Forgot, Add 1TB HDD: AU$64 - total: AU $594
That makes, in US Dollars, $528, for something with quite a bit of grunt to it. Add any additional components and shipping and you should still be well under your budget.