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Under £100 for a 470 is a bargain.GTX470 i sold one of my mine for less then £100 or possible a hd5870
4890 beats a 4870 though, they're probably hideously close in terms of performance. Just the 4890 lacks DX11 and will eat more watts.I would get a 6850 over the 4890 even though it would be cheaper as it lacks DX11 and is slower.
Here's a benchmark between a 4870/6850 to give you a rough idea between performance.
http://www.anandtech.com/bench/Product/304?vs=291
 
	