Best gaming pc for £800

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor (£163.90 @ Amazon UK)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler (£25.45 @ Scan.co.uk)
Motherboard: ASRock Z97 PRO4 ATX LGA1150 Motherboard (£89.99 @ Ebuyer)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1866 Memory (£61.01 @ CCL Computers)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£39.50 @ Amazon UK)
Video Card: Sapphire Radeon R9 280X 3GB Vapor-X Video Card (£236.75 @ Aria PC)
Case: Corsair 200R ATX Mid Tower Case (£39.95 @ Amazon UK)
Power Supply: XFX 650W 80+ Bronze Certified Semi-Modular ATX Power Supply (£62.69 @ CCL Computers)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£10.84 @ Dabs)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) (£68.14 @ CCL Computers)
Total: £798.22
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-26 16:18 BST+0100)
 


+1 but he can always go for Windows 8 as well if he prefers to.