Here's what I recommend you :
PCPartPicker part list:
http://uk.pcpartpicker.com/p/1CVX0
Price breakdown by merchant: http://uk.pcpartpicker.com/p/1CVX0/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/1CVX0/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor (£83.99 @ Aria PC)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard (£59.00 @ Amazon UK)
Memory: GeIL Enhance CORSA 8GB (2 x 4GB) DDR3-1600 Memory (£40.99 @ Amazon UK)
Storage: Hitachi Ultrastar 1TB 3.5" 7200RPM Internal Hard Drive (£37.99 @ Aria PC)
Video Card: Sapphire Radeon HD 7790 1GB Video Card (£95.97 @ Dabs)
Case: Cooler Master Force 500 ATX Mid Tower Case (£32.87 @ Scan.co.uk)
Power Supply: XFX 450W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£34.99 @ Maplin Electronics)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£12.78 @ Amazon UK)
Total: £398.58
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-09-12 18:52 BST+0100)