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Thanks for all the suggestions, advice chaps!

I just installed the 2Gb MSI N560GTX Ti Twin Frozr which seems a well made card and comes slightly OC'd over the standard settings. The Asus 1Gb 560Ti Direct CU has slightly higher clock speeds out of the box but the difference is only marginal. The MSI Afterburner software was installed so I can tweak it to match if necessary.

With my 2.7Ghz Duel Core system I don't expect to see a massive improvement other than running with some of the eyecandy enabled in MP and more Video settings at high with a few at medium/Low (Buildings, trees, etc).

Benchmarking isn't something I do - I tend to make judgements in game but I did check FPS on the Black Death Track and the results are marginally better but not significantly better - more a case of less sudden large fluctuations (except when the bomb explodes and sudden dip to 9fps for a brief moment). In game it seemed better and used the Quick Mission 'Intercept Bombers over London' for comparison using High settings and Shadows, clouds enabled.

After the next update I will be looking to upgrade to an i5 - 2500K 8Gb. At the moment I am happy with the SP and MP performance on my current system.