It's honestly a toss up. NVIDIA has been historically more stable/reliable in terms of performance, whereas AMD cards vary (sometimes wildly) in performance based on drivers and games. A pair of 780 Ti's is definitely a force to be reckoned with, and so are a pair of 290x lol
**Personally** I prefer NVIDIA, and would **personally** get a pair of 780 Ti's, but it's up to you. Both will give you very impressive performance, and a setup with a pair of either of them is able to run High/Ultra in 4k around 50-60 FPS.
Also, if this is strictly for gaming, save yourself a
ton of money and skip the 16GB of Dominator RAM, it's a huge waste of money. You can get 1600 or 1866 CAS9 G.Skill or Corsair RAM for half the price, and even cheaper if you get 8GB, as 16GB is overkill for gaming already.
If you do stick with NVIDIA, an 850w is perfect:
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i7-4770K 3.5GHz Quad-Core Processor (£233.99 @ Aria PC)
Motherboard: Gigabyte GA-Z97X-Gaming 5 ATX LGA1150 Motherboard (£112.60 @ Ebuyer)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 Memory (£59.91 @ Ebuyer)
Storage: Samsung 840 EVO 250GB 2.5" Solid State Drive (£91.99 @ Amazon UK)
Storage: Western Digital BLACK SERIES 1TB 3.5" 7200RPM Internal Hard Drive (£55.93 @ Amazon UK)
Video Card: EVGA GeForce GTX 780 Ti 3GB Superclocked Video Card (2-Way SLI) (£529.94 @ Amazon UK)
Video Card: EVGA GeForce GTX 780 Ti 3GB Superclocked Video Card (2-Way SLI) (£529.94 @ Amazon UK)
Case: Corsair 450D ATX Mid Tower Case (£89.99 @ CCL Computers)
Power Supply: EVGA 850W 80+ Gold Certified Fully-Modular ATX Power Supply (£92.73 @ Amazon UK)
Optical Drive: Lite-On iHDS118-04 DVD/CD Drive (£10.78 @ Scan.co.uk)
Operating System: Microsoft Windows 8 (OEM) (64-bit) (£72.88 @ Ebuyer)
Total: £1880.68
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-06-04 22:07 BST+0100)