+1 to the 5870. It uses much less power than the GTX400 series cards so a lesser power supply will do more ATI cards compared to Nvidia cards. You will need a good 750W PSU to be able to crossfire 5780s tho. The cheapest good one I've seen is the Seasonic S12D 750, at around £93 on ebuyer.
And the 5870 is faster and better than the 470 gtx but unfortunately that is just a scam to promote amazon. That card will never be in stock( I know I wanted to buy it and contacted amzon for an indication when it will be back and they said probably never). As you might see 5870 are actually far more expensive.
With a £1000 budget there are definitely ways to get a card as expensive as a 5870 into the build, and if gaming is the most important and stressing task it does then you should probably go for the best graphics card you can get and have the rest of the system be based around AM3 or 1156.
If you're going to stick with 1366 then the card you'll want is the 5850 as it is around the same price as a GTX 285, uses less power and outperforms it in most benchmarks I've seen.