See
http://www.tomshardware.com/reviews/radeon-r9-295x2-crossfire-performance,3808-4.html
You need 2 x R9 295X2's in Xfire. Or maybe 4 x gtx980's might be better. Apparently they scale well.
To do that you need a mess of pcie lanes. So X99 and an i7 5930k
Something like -
CPU: Intel Core i7-5930K 3.5GHz 6-Core Processor ($549.97 @ Directron)
CPU Cooler: Corsair H100i 77.0 CFM Liquid CPU Cooler ($79.99 @ NCIX US)
Motherboard: ASRock X99 Extreme11 ATX LGA2011-3 Motherboard ($645.99 @ SuperBiiz)
Memory: G.Skill Ripjaws 4 series 32GB (4 x 8GB) DDR4-2400 Memory
Storage: Samsung 840 EVO 250GB 2.5" Solid State Drive ($149.99 @ B&H)
Storage: Seagate 4TB 3.5" 5900RPM Hybrid Internal Hard Drive ($169.99 @ Amazon)
Video Card: MSI GeForce GTX 980 4GB Twin Frozr Video Card (4-Way SLI) ($579.99 @ Newegg)
Video Card: MSI GeForce GTX 980 4GB Twin Frozr Video Card (4-Way SLI) ($579.99 @ Newegg)
Video Card: MSI GeForce GTX 980 4GB Twin Frozr Video Card (4-Way SLI) ($579.99 @ Newegg)
Video Card: MSI GeForce GTX 980 4GB Twin Frozr Video Card (4-Way SLI) ($579.99 @ Newegg)
Case: Corsair 900D ATX Full Tower Case ($259.99 @ NCIX US)
Power Supply: SeaSonic X Series 1250W 80+ Gold Certified Fully-Modular ATX Power Supply ($218.99 @ SuperBiiz)
Optical Drive: Asus BW-16D1HT Blu-Ray/DVD/CD Writer ($64.99 @ Newegg)
Operating System: Microsoft Windows 8.1 Pro (OEM) (64-bit) ($128.98 @ OutletPC)
Wireless Network Adapter: Asus PCE-N53 802.11a/b/g/n PCI-Express x1 Wi-Fi Adapter ($24.99 @ Newegg)
Total: $4613.83