Anyways, here is build
Top notch CPU for gaming, very cool high end video
So it's even under your budget, OS not included.
If you're still going for prebuilt just look for these parts (important are: CPU, video, motherbrd)
PCPartPicker part list:
http://pcpartpicker.com/uk/p/nPIm
Price breakdown by merchant: http://pcpartpicker.com/uk/p/nPIm/by_merchant/
Benchmarks: http://pcpartpicker.com/uk/p/nPIm/benchmarks/
CPU: Intel Core i5-3470 3.2GHz Quad-Core Processor (£143.38 @ Amazon UK)
Motherboard: ASRock H77 Pro4/MVP ATX LGA1155 Motherboard (£69.99 @ Amazon UK)
Memory: G.Skill Sniper Low Voltage Series 8GB (2 x 4GB) DDR3-1600 Memory
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive (£41.98 @ Scan.co.uk)
Video Card: MSI Radeon HD 7850 2GB Video Card (£149.74 @ Scan.co.uk)
Case: Cooler Master RC-361-KKN1 (Black) ATX Mid Tower Case (£28.07 @ Scan.co.uk)
Power Supply: SeaSonic S12II 520W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£63.61 @ Amazon UK)
Optical Drive: Samsung SH-222BB/BEBE DVD/CD Writer (£11.96 @ Scan.co.uk)
Total: £508.73
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2012-11-16 23:31 GMT+0000)