If you want something really small, you could get this :
PCPartPicker part list:
http://pcpartpicker.com/p/1v5po
Price breakdown by merchant: http://pcpartpicker.com/p/1v5po/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1v5po/benchmarks/
CPU: Intel Core i5-4430 3.0GHz Quad-Core Processor ($174.99 @ Amazon)
Motherboard: ASRock B85M-ITX Mini ITX LGA1150 Motherboard ($85.98 @ SuperBiiz)
Memory: Crucial Ballistix Smart Tracer 4GB (2 x 2GB) DDR3-1600 Memory ($35.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($62.99 @ NCIX US)
Video Card: Sapphire Radeon HD 7950 3GB Video Card ($199.99 @ Newegg)
Case: Cooler Master Elite 120 Advanced (Black) Mini ITX Tower Case ($35.99 @ Microcenter)
Power Supply: Corsair Builder 600W 80 PLUS Certified ATX12V Power Supply ($39.99 @ Newegg)
Total: $635.92
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-22 18:39 EDT-0400)