Compare the 8800GS to the 9600GT: seems to me that they perform about the same, and both on par with the 8800GTS 320, although the GS unsuprisingly seems to suffer above 1280res. Both around the £100 mark.
Personally I'd take the 3850 512mb over both the 8800GTS 320mb and 8800GS.
Also I've had a 8800GTS 640mb before and I still prefer the 3850, without any vmods when both are clocked to their limits the 3850 is faster in almost everything except Crysis....and once you start loading up AA in Crysis the 3850 pulls far ahead of the 8800GTS 640mb