PCPartPicker part list:
http://uk.pcpartpicker.com/p/PXAf
Price breakdown by merchant: http://uk.pcpartpicker.com/p/PXAf/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/PXAf/benchmarks/
CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor (£167.99 @ Aria PC)
CPU Cooler: Zalman CNPS5X Performa CPU Cooler (£14.99 @ Novatech)
Motherboard: ASRock Z77 Extreme4 ATX LGA1155 Motherboard (£103.46 @ Amazon UK)
Memory: Corsair XMS3 8GB (2 x 4GB) DDR3-1600 Memory (£32.99 @ Amazon UK)
Storage: Crucial M4 128GB 2.5" Solid State Disk (£88.29 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£49.94 @ CCL Computers)
Video Card: Sapphire Radeon HD 7970 GHz Edition 3GB Video Card (£341.40 @ Amazon UK)
Case: Silverstone SST-RL01B-USB 3.0 (Black) ATX Mid Tower Case (£51.08 @ Overclockers.co.uk)
Power Supply: SeaSonic 650W 80 PLUS Gold Certified ATX12V / EPS12V Power Supply (£105.50 @ Amazon UK)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer (£11.13 @ Amazon UK)
Monitor: Asus VE247H 23.6" Monitor (£142.81 @ Amazon UK)
Monitor: Asus VE247H 23.6" Monitor (£142.81 @ Amazon UK)
Total: £1252.39
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-04-11 15:36 BST+0100)
i'm not sure which company is the best on building rig, why not DIY? it's free and it's fun.