I've been looking for a 5850 for the past few months, but as prices spiral up towards the £300 mark, the 5870 is looking more and more tempting. The Sapphire 5870 Vapor-X has a non-reference cooler which helps keep it quiet and cool - it looks awesome too!
Next cheapest I can find is overclockers at £329.99.
Here's the gubbins:
The SAPPHIRE HD 5870 Vapor-X is based on the latest graphics architecture from the ATI division of AMD, the second generation of GPU to be built in its 40nm process. It supports the advanced graphical features of DirectX 11, and delivers spectacular video clarity, speed and visual effects, including over multiple monitors with the new ATI Eyefinity mode. Using ATI Stream technology, accelerate even the most demanding applications and do more than ever with your PC. The first to deliver full support for Microsoft DirectX 11, these GPUs enable rich, realistic visuals and explosive HD gaming performance so you can dominate the competition.