i5 750, EVGA P55 Micro, 4GB DDR3, Fractal Define Mini Case, TX650 PSU
 

Selling these parts all assembled together in the case:

Intel Core i5 750 Processor
EVGA P55 Micro SLI Motherboard (one of the few mATX boards that support SLI)
G.SKILL 4GB 2X2GB DDR3-1333 RAM
Corsair TX650 650W Power Supply
Fractal Define Mini mATX Case with 3x 120mm Noctua fans installed

All this system needs is a graphics card and hard drive to be fully functional. The processor is still good enough for any gaming machine.

Asking $300.