Here is a nice guide for a $500 "Steam Box"
http://www.digitaltrends.com/computing/how-to-build-your-own... Great pictures for assembly - components as follows:
Processor: Intel Core i3-3220 ($130)
Motherboard: Gigabyte GA-H61M-HD2 Micro-ATX ($55)
RAM: Kingston Value 4GB DDR3 1066 Low-Profile ($30)
Video Card: AMD Radeon HD 7770 1GHz Edition ($115)
Hard Drive: Western Digital Blue 500GB 7200RPM ($60)
Power Supply: Antec NEO ECO 400-Watt ($40)
Case: Antec New Solution VSK-3000 ($30)
Cooler: Cooler Master GeminII M4 ($30)