Dell has recently released the alpha - small form factor PC. It is smaller than PS4 and Xbox One but more powerful and upgradable (except gpu). There are three base models - with core i3, i5 and i7 but only i5 model available on ebay au.
Some reviews:
http://www.gizmodo.com.au/2014/11/alienware-alpha-review-clo…
http://www.pcworld.com/article/2850889/alienware-alpha-revie…
http://www.tomsguide.com/us/alienware-alpha,review-2519.html
The closest mini-itx build will probably cost you around $920 (does not include assembly)
Seagate 3.5" Barracuda 1TB HDD $62.00
Patriot Signature 8GB Kit (4Gx2) DDR3-1600 $87.00
64bit Microsoft MS Windows 8.1 OEM $112.00
ASRock H81M-ITX $75.00
Coolermaster Elite 110 Black Mini-ITX Case $45.00
Gainward 2G GTX 750TI PCI-E VGA Card $158.00
Intel Core i5 4590 3.3GHz 6MB $235.00
EVGA 430W (100-W1-0430-KR) 430Watt 80Plus Power Supply Unit $49.0
XBOX 360 Controller $36
TP-LINK ARCHER T2U Wireless AC600 Dual Band USB Adapter 802.11ac $35
EDIT: updated title to be more accurate - this use gtx 860m which is comparable to 750ti.
they really are damn tiny