Thank you Feuer Frei! and BigBang Theory for your advice and recommendations. I went to the hardware compare website, very informative, clear concise definitions and descriptions of the different benchmarks. Very good comparisons between the two cards.
All this now lives me with a conundrum. Here are my options;
GTX 460 on offer for £80, the GTX 570 for £230, ATI 6870 - 1 GB GDDR on offer for £139.00, and finally the the ATI 6950 2GB for £200.
As with many things there's the "you pays your money you pay's your choice' trade off between budget/performance. Highly tempted to go for the 6950 but my head say's get the 460 (not planning to play Arma2).
Thank you all again for your advice, makes for a more informed decision.