I was in the same position as you about a week ago. This site goes
through lots of manufacturers who have made 6600GT's:

http://www.anandtech.com/video/showdoc.aspx?i=2295&p=1

Note that the Leadtek card reviewed on here is a slightly different
model to the one on overclockers.co.uk, but the one on overclockers is
more up to date.

I bought the Leadtek Winfast A6600GT TDH and it really does run games
smooth at high res and high details. I had some graphics problems with
a couple of games but sorted that today on here thanks to DaveL. To
avoid this make sure you get the 71.20 driver from guru3d.com:
http://downloads.guru3d.com/download.php?det=931