This is about as good as it gets for trying to do a rendering rig on such a budget:
PCPartPicker part list:
http://pcpartpicker.com/p/1rVCs
Price breakdown by merchant: http://pcpartpicker.com/p/1rVCs/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1rVCs/benchmarks/
CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor ($92.13 @ TigerDirect)
Motherboard: MSI 970A-G46 ATX AM3+ Motherboard ($74.99 @ Amazon)
Memory: Corsair 16GB (2 x 8GB) DDR3-1600 Memory ($113.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.98 @ Outlet PC)
Video Card: PowerColor Radeon HD 7870 XT 2GB Video Card ($233.98 @ Newegg)
Case: Gigabyte GZ-F5HEB ATX Mid Tower Case ($28.12 @ Amazon)
Power Supply: EVGA 500W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($29.99 @ Microcenter)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Total: $648.17
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-14 14:35 EDT-0400)