6600 GT's RAM and GPU are clocked higher than 6800.. Dude, 6600GT beats 6800 in many tests.. However... I would suggest neither.. Go for 7600 GT if you can afford it at the moment.. Or wait for some more time.. The prices are expected to dip...
The vendor's probably giving 6800 at a lower price just to clear his existing stocks... No one buys a 6800 these days.... or even 6600 ...
If you HAVE to choose between these two, go for the 6600 GT.