Graphics Card for under £150 (Under £100 preferred)?

Hi. I have recently built a new PC, which is currently using an old Graphics card, and I need an upgrade. I'm not very experienced when it comes to Graphics cards, and I was wondering what would be a good card to run games such as Far Cry 2 and Crsis (And others) at decent settings for under £150, preferably under £100.

I have looked around and I just don't know which to choose. My friends reccomend the ATI 4850 and the NVIDIA 8800GT/GS.

I don't prefer ATI or NVIDIA, All I know about the two is that ATI is generally cheaper. So, do any of you have any opinions?