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I think you'd be lucky to get that for a GTX570 with new cards now on the market. 580's have been going for close to the £200 mark, sometimes a little less.
Yeah the GTX570's are still good cards but I would probably be leaning towards putting money to upgrade the Q6600. Yes its still a good little CPU when overclocked but any sort of upgrade from your current 570 will be holding it back. May a nice second hand Sandy Bridge i5-2500K/Mobo/8GB DDR3 RAM instead?