I don't work in AUD, I work in pounds, but I presume that $850 is around £500, so here is what I would suggest:
Case:Cheapest Mid Tower case that you can get your hands on as long as it has front and rear fans.
Motherboard: Gigabyte GA-970A-DS3
CPU: AMD FX 8120
RAM: 8GB Corsair XMS3 @1600MHz and 1.5v
GPU: AMD Radeon HD 7770
Optical Drive: AD-5280S-0B
OS: Windows 7 (or Zorin OS(Linux))
HDD: Seagate Barracuda 500GB, or just get a cheap laptop drive. They tend to be the cheapest things you can buy.
PSU: Corsair CP-9020046
All added up, that costs £532.46. In Australia, you probably get better value. So, you could get a better case/motherboard/graphics card/CPU. And without Windows, you can definitely get the FX-8350 and a GTX-660 in your budget. Find them where they are cheapest and make sure that you shop arond.