£400 - 500 mainly gaming pc build

Solution
I would save that ten extra pound and get this:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-6300 3.5GHz 6-Core Processor (£88.79 @ Aria PC)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard (£68.97 @ Amazon UK)
Memory: GeIL Enhance CORSA 8GB (2 x 4GB) DDR3-1600 Memory (£40.99 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£45.59 @ Aria PC)
Video Card: Sapphire Radeon HD 7870 GHz Edition 2GB Video Card (£163.30 @ Amazon UK)
Case: NZXT Source 210 Elite (Black) ATX Mid Tower Case (£39.58 @ Scan.co.uk)
Power Supply: XFX 550W 80 PLUS...
PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor (£59.99 @ Aria PC)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard (£68.97 @ Amazon UK)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory (£50.42 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£45.59 @ Aria PC)
Video Card: Sapphire Radeon HD 7870 GHz Edition 2GB Video Card (£163.30 @ Amazon UK)
Case: NZXT Source 210 Elite (Black) ATX Mid Tower Case (£39.58 @ Scan.co.uk)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£49.86 @ Scan.co.uk)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer (£13.15 @ Amazon UK)
Total: £490.86
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-12 06:05 BST+0100)

-Overclockable if you add a future aftermarket cpu cooler.
-If you can spend an extra £20, grab the FX-6300.
 
I would save that ten extra pound and get this:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-6300 3.5GHz 6-Core Processor (£88.79 @ Aria PC)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard (£68.97 @ Amazon UK)
Memory: GeIL Enhance CORSA 8GB (2 x 4GB) DDR3-1600 Memory (£40.99 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£45.59 @ Aria PC)
Video Card: Sapphire Radeon HD 7870 GHz Edition 2GB Video Card (£163.30 @ Amazon UK)
Case: NZXT Source 210 Elite (Black) ATX Mid Tower Case (£39.58 @ Scan.co.uk)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£49.86 @ Scan.co.uk)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer (£13.15 @ Amazon UK)
Total: £510.23
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-12 08:07 BST+0100)

-Doubled your RAM and HDD for not much more.
 
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