I have just bought the HPx20LED monitor and although the above card still works well, I wondering whether I would see any noticeable improvement by getting a newer grahics card. The amount I have to spend would be approx £150.00 although I maybe able to stretch to £200.00 if there is a marked improvement between the cards performance.
Strategy games don't normally need massive graphic processing power, so are unlikely to benefit from an upgraded card. First person shooters, however, do tend to benefit.
Something like an HD6850 should show you some differences - better frame rates, or improved overall graphics - and fits you £150 budget. The HD6870 is a (slightly) better card, and comes in at just under £200.
Be a little careful about how much memory is on the graphic card you choose - whatever you have on the card can end up being effectively removed from your system RAM. Make sure you have plenty of available RAM when you are gaming before considering putting in a 1GB card!