PCPartPicker part list:
http://uk.pcpartpicker.com/p/24pw1
Price breakdown by merchant: http://uk.pcpartpicker.com/p/24pw1/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/24pw1/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor (£83.99 @ Aria PC)
Motherboard: Asus M5A97 LE R2.0 ATX AM3+ Motherboard (£54.84 @ CCL Computers)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£59.99 @ Amazon UK)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£46.42 @ CCL Computers)
Video Card: Asus Radeon HD 7790 2GB Video Card (£94.60 @ Dabs)
Case: Cooler Master Force 500 ATX Mid Tower Case (£32.87 @ Scan.co.uk)
Power Supply: Corsair CX 430W 80+ Bronze Certified Semi-Modular ATX Power Supply (£37.27 @ Amazon UK)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer (£13.15 @ Amazon UK)
Total: £423.13
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-11-18 21:19 GMT+0000)
This is close to the best computer you can get at this price. You might not reach 60fps, but you can get smooth gameplay. You will need to expand your budget if you need a monitor, windows, etc. or get a lower-tier cpu or gpu. If you have a hdd already, you can save money there, too. 8gb of ram is pretty much the standard for gaming in next-gen games, and 1tb of storage is a sweet spot. The gpu could be potentially downgraded to a 7770 Ghz edition if you really need the system at £400.