Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
If you don't want 2nd hand currently there is a
EVGA GeForce GTX 480 1536MB GDDR5 PCI-Express Graphics Card (015-P3-1480-KR) [015-P3-1480-KR]
This Week Only Offer
was £184.99 inc VAT
£167.99 inc VAT
£139.99 ex VAT
http://www.overclockers.co.uk/showproduct.php?prodid=GX-124-EA&groupid=701&catid=1914&subcat=1830
Which will be a nice upgrade for new at that sort of price range.
Benchmark Results:-
AMD Radeon HD 5850 - 725MHz/4000MHz vs NVIDIA GeForce GTX 480 - 700MHz/1401MHz/924MHz
http://www.anandtech.com/bench/Product/295?vs=309
How about a 2nd 5850? Is your mboard xfire capable?
You could get a massive increase for < £80
There is a Sapphire 6950 on This Week Only for £150, depending on resolution you could get about a 20% performance increase.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-299-SP
Was not on offer when I posted earlier but was looking for it and now dropped to £150 again it's a bargain.