im will be useing win 7 just so you know i did have a 8800gtx but i got the bsod that you cant fix
i was looking at the ati 5850 0r a 4870x2 or two 4850
just dont know what to do
5850 gets my vote. 4870x2 is faster, but needs a bigger PSU. 2x4850 are slower or about the same as a 5850, but consume almost double the wattage of the 5850.
Also the 5850 has more features and is DX11 compatible, the rest aren't.
The 5850 is the best choice of the suggestions above because its newer, more efficient, and offers DX 11. (However, depending on the size of your monitor you might not actually need this much power to max out games--post it if you want advice there)
A single card is usually a better option than double cards. Also, by getting a single good card now you have the option crossfire it as an upgrade later.
You should be able to find a 5850 for about £245. The next level up would be a 5870 at £330. If you want to spend less I believe the 5830 will be launching soon.
Finally, have you considered overclocking your processor? After getting a 5850 it will become the weakest link in your computer.