In your budget, go to my link in my sig, it's much better than the build you have, It has 8GB RAM, i5 3570k and a radeon 7870 in it, and all under you're budget.
Edit: Here you go!
CPU: i5 3570k £177http://www.ebuyer.com/349029-intel
GPU: Radeon 7870 2GB GDDR5 £200http://www.overclockers.co.uk/show
Case: Coolmaster Elite 430 £35http://www.amazon.co.uk/Cooler-Mas
Mobo: GA-Z77-D3H £78http://www.ebuyer.com/351598-gigab
OS: Windows 7 64 bit £73http://www.amazon.co.uk/gp/offer-l
HDD: 1TB Black Caviar WD 7200rpm £85http://www.ebuyer.com/193712-wd-1t
DVD Drive: Sony AS-5280S £13http://www.ebuyer.com/278111-sony-
PSU: XFX 550watt Core edition £50http://www.amazon.co.uk/gp/offer-l
RAM: Corsair 2X4GB 1600MHZ £39http://www.ebuyer.com/274036-corsa
TOTAL=£750 (If you go down the 7850 route you can overclock to around standard 7870 speeds and get a SSD with the £40 left over)