I currently have 2x 1gb Sapphire HD4870s in crossfire config. With a budget of £400 is there a card/s worth buying to top these, remembering your advice that its not worth upgrading unless you get 3x the performance.
Well the AMD high end come out in 2-3 weeks. 6970. Which isnt a dual card, the dual card is the 6990 due to how they renamed things. The 5870 is about 5% slower in general than 2x 4870s. The 6970 is suppose to be 25-40% faster than the 5870. So it would be 20-35% faster than your 2 4870's in all likelyhood. Nvidia is also releasing a GTX 580 somewhere between December and February, however than looks to be +/- 5% of the 6970 in speed, so that wouldnt be much faster.