You might be able to squeeze in a 6 core. But 10 sec in mental ray is probably impossible depending on your lighting complexity. What do you have now and how long does it take?
PCPartPicker part list:
http://pcpartpicker.com/p/1nphl
Price breakdown by merchant: http://pcpartpicker.com/p/1nphl/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1nphl/benchmarks/
CPU: Intel Core i7-3930K 3.2GHz 6-Core Processor ($499.99 @ Microcenter)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.99 @ Newegg)
Motherboard: ASRock X79 Extreme3 ATX LGA2011 Motherboard ($199.99 @ Microcenter)
Memory: Crucial Ballistix Sport 16GB (4 x 4GB) DDR3-1600 Memory ($127.11 @ TigerDirect)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.98 @ Outlet PC)
Video Card: EVGA GeForce GTX 660 2GB Video Card ($169.99 @ Newegg)
Case: Corsair 300R ATX Mid Tower Case ($57.99 @ Microcenter)
Power Supply: Rosewill Hive 650W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($49.99 @ Newegg)
Total: $1195.03
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-02 10:35 EDT-0400)