Not sure you'll get B4 at 30 FPS, but 500 with OS is really cutting it. Almost cheaper to buy an OEM computer and add a video card.
PCPartPicker part list:
http://pcpartpicker.com/p/241cK
Price breakdown by merchant: http://pcpartpicker.com/p/241cK/by_merchant/
Benchmarks: http://pcpartpicker.com/p/241cK/benchmarks/
CPU: AMD FX-4300 3.8GHz Quad-Core Processor ($99.99 @ Amazon)
Motherboard: Gigabyte GA-78LMT-S2P Micro ATX AM3+ Motherboard ($44.98 @ OutletPC)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($59.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($59.94 @ Amazon)
Video Card: Sapphire Radeon HD 7770 GHz Edition 1GB Video Card ($85.38 @ Newegg)
Case: Apex PC-389-C ATX Mid Tower Case ($19.99 @ Newegg)
Power Supply: Corsair Builder 430W 80+ Bronze Certified ATX Power Supply ($19.99 @ Microcenter)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.00 @ Amazon)
Total: $494.25
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-11-17 20:46 EST-0500)