Value too be had is either with the 460 or 470. Value and Nvidia could rarely been said in the recent past, but it is the case at the moment. Not like ATI is bad value at the moment with the HD5850 but with the 470 faster for not a great deal more and a 460 almost as fast while considerably less, Nvidia is the logical choice.
Would not consider the 5870 at the moment considering the 5850 can overclock to and surpass the 5870 stock speeds.
The differences between 460 and 5850, and those between 5850 and 470 are very similar. 10-15% performance increases from going from one to another and about £30 extra each. IMO worth considering either card depending on the budget.