The 4850 is a good card, Ben has one and he seems to run everything fine. 2 others to look at would be the GTS250 - basically the same price and performance as the ATi and the 4870 - £10 more but a better card.
Jamie, if you can get the latest ATI cards you will be future proof. Or wait until Nvidia release their cards, ATI will respond with a price reduction on theirs. Better for consumer if competition is around.