I add this list for you £520.60, if you have to stay within 400-500pounds with OS, then you can take out the cooler.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/2RvHe
Price breakdown by merchant: http://uk.pcpartpicker.com/p/2RvHe/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/2RvHe/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor (£80.39 @ Aria PC)
CPU Cooler: Zalman CNPS10X OPTIMA CPU Cooler (£16.87 @ CCL Computers)
Motherboard: Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard (£35.94 @ CCL Computers)
Memory: Patriot Viper 3 8GB (2 x 4GB) DDR3-1600 Memory (£59.99 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£39.98 @ Aria PC)
Video Card: Gigabyte Radeon HD 7870 2GB Video Card (£136.34 @ CCL Computers)
Case: Cooler Master K280 ATX Mid Tower Case (£31.17 @ Amazon UK)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply (£37.60 @ Amazon UK)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£11.92 @ CCL Computers)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) (£70.40 @ CCL Computers)
Total: £520.60
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-02-11 21:25 GMT+0000)