I would personally avoid the 6850, great card, great price, just not enough horsepower. I would look at the 6870 (not much more in cash anyways about 30 pounds difference on Microdirect.co.uk) and the 6950. I would also however look at the 560 TI version as well as it is in the same price range as the 6870.
Also I found that the brand spanking new 660 TI is about 40 pounds ($242 pounds) more then then 6950($202 pounds), thats with the VAT included and the 660 TI performance is stellar and way beyond that of the 6950. You can also keep the 7850 and 7870 in mind as well.
That all being said, what NyteOwl said is right. Base your choice on the amount of money you have(or can save) and what performance levels you want to achieve in the game you play. Your Power Supply is a very good unit and could power as high as a GTX 680 or 7970HD without breaking a sweat and still have power left over.
If you are looking to save money, I would look at the 6870, 7850, and the 560 TI. Any of these cards will perform well in any of the games you listed.
If you can save or wait a bit more, I would be looking squarely at that 660 TI. The price/performance level is extremely hard to beat. And it does that with A TDP of 150 watts too