You should get a Radeon HD4850 - thats only about £90. Amazing card
Link: http://www.amazon.co.uk/Sapphire-HD4850-51...0662&sr=8-1
If you want to go a step up - The radeon HD4870 is the next one up - http://www.amazon.co.uk/Sapphire-11133-03-...0951&sr=1-3 - pricetag: £120
Then there's...