the the 560 is the most powerful card in there but the 5850 isn't far behind. If you don't list the prices, its hard to say whats worth the money.
generally the 6850 can almost be had for 100 pounds for some of the best price and performance you can get.
Sorry. Prices and manifacturers (poor spelling, sorry)
Gtx 560 £130 - £150 (asus)
5850 £110 (his)
5770 £130 (powercolor)
Prices are from ebay, but i would buy from amazon if the seller rating is low)