I put together this build a couple months ago. It is still a solid gaming build for under £500. I noticed that there are a few things in here that you will not need or already have but this could give you a good idea of what components you can consider in your build.
These prices may be off by a little bit.
ARIANET Synergy Black Midi Tower Chassis w/ 450W Power Supply #47026 £ 16.99
PowerColor Radeon HD 6850 1024MB GDDR5 PCI-Express Graphics Card #47805 £ 82.99
GIGABYTE GA-970A-DS3 AMD 970A (Socket AM3+) DDR3 PCI-Express ATX Motherboard #48467 £ 42.99
AMD Bulldozer FX-4100 3.60GHz (3.80GHz Turbo) Socket AM3+ 4-Core Processor #46818 £ 69.99
8GB Exceleram Value Series LP E30113A (2x4GB) 1333 (PC3-10666) 240-Pin DDR3 SDRAM CAS 9-9-9-24 1.5V #48306 £ 23.99
320GB Western Digital AV-GP 24/7 SATA II Hard Drive - CLEAN PULL #48044 £ 41.24
Microsoft Windows 7 Home Premium 32 Bit, SP1, Operating System, Single, - OEM #38271 £ 57.50
Sony 24x Internal DVD-RAM+/- Writer SATA Black Bare Drive #48288 £ 10.91
19" AOC E950SWDA Widescreen LED Monitor - Black #47597 £ 55.99
Basket Total £ 402.59
Delivery Costs: Approx. Delivery £ 8.17
VAT £ 82.15
Total £492.91
Everything needed for less that £500.