Well, a £200 budget does give you a card that I would class as 'super'!
Looking at your board, it appears to support crossfire, so i'd highly recommend that you go down the ATI route. Even if you only put one card in for now, you can always add to it later.
You might just about be able to pick up an HD5850 for £200 if you shop around. An HD5770 is well inside your budget at around £120, but does not perform quite as well. With the games that you play, I'm not sure how much difference you'd notive between them though.