It's a tough one, but I think I'm going to go for the Sapphire ATI Radeon HD 5870 Vapor-X GFX card - Looks to be the third best in the 5870 series, with the XFX and Powercolor in the lead...
I would seriously recommend you look at getting a 5850 over a 5870, 5870s just aren't worth the £100+ difference in monies.
You can overclock a 5850 to 5870 performance and beyond (yeah you can overclock the 5870 too but it's still not £100+ more performance).