I built my computer similar to these guidelines below, with slight differences such as 16 GB RAM instead of 8, a cheaper hard drive before the Thailand flooding, no graphics card, and Windows 7 Professional. But this build below outlines a fine gaming computer with mostly second-best materials.
Intel Core i7-2600K - $320
GIGABYTE Z68 Motherboard - $170
Hitachi Deskstar 1TB 7200 rpm hard drive - $70
Cooler Master Elite 430 Black Mid Tower - $39
Thermaltake 600W PSU - $60
Any DVD burner - $20
Radeon HD 6850 1GB - $150
DDR3 8GB 1600MHz DUAL CORSAIR - $30
Windows 7 Home Premium 64bit OEM - $80
Asus wireless-n adapter card - $30
Intel Core i7-2600K - $320
GIGABYTE Z68 Motherboard - $170
Hitachi Deskstar 1TB 7200 rpm hard drive - $70
Cooler Master Elite 430 Black Mid Tower - $39
Thermaltake 600W PSU - $60
Any DVD burner - $20
Radeon HD 6850 1GB - $150
DDR3 8GB 1600MHz DUAL CORSAIR - $30
Windows 7 Home Premium 64bit OEM - $80
Asus wireless-n adapter card - $30