6600 GT comes with DDR 3 memory and more pipelines than a 6600 plain. A plain 6600 is the low end model of the GT chipset which is only good for low resolution gaming. I dont suggest a 6600 plain unless u r very tight on budget. Go for the 6600GT from BIG , it is for 8500+taxes in delhi. a very very good deal there.
ask them for BIG 6600GT 128 MB DDR PCI-EXPRESS thats it .. also check the latest hardware prices and quotes sticky of this forum for all the latest pricing of BIG cards available. that should give you some idea.