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A 5870 should be a decent bump in performance, however, a 5850 will probably give you a bump, and well, it remains to be see if they overclock as high as 5870's do, but they overclock a fair bit which gets them pretty close to a 5870 for £100 less.
You be best to wait a few weeks for the 5870x2 to get released...As title really,
What about the latest offerings from Nivida or ATI, don't care about price