I would suggest if buying right now a 5970. You get GTX470 SLI performance on one card for about 50 quid less.
However, if you're buying in September, keep your options open because the ATI refresh will be coming along at some point, which will ruffle things up a bit and result in price drops =)
However, if you're buying in September, keep your options open because the ATI refresh will be coming along at some point, which will ruffle things up a bit and result in price drops =)