from reviews i've seen:
BFG NVIDIA GeForce 8800 GTS OC 320MB PCIe http://www.bfgtech.c...bfgr88320gtsoce.aspx
hightly rated for under £200.
this is an overclocked card so be careful to get the right model - otherwise it might not be such a great card.
if you aren't bothered about directx10, and i don't see why anyone should be at the moment when none of the current cards seem to be up to the task of playing directx10 games, then you could go for something a bit older and therefore cheaper.
if you want directx10 then i'd wait until next year before buying a new card - there might be something available by then that will be worth buying. or maybe we'll have games that are a bit better tuned to work with the cards that are already for sale.