Nope, the 6600GT has a competely different chip from the 6800 series.
The 6800 series is all 1 chip, unless they've started making a seperate 6800 chip by now.
And then the 6600 and 6600GT share the same chip. The 6200s used to, but have since had their own chip developed.
BTW, you wouldn't want to make the 6600GT into a 6800, the 6600GT is already a faster chip, however the 6800s have much higher memory bandwidth, so if you did make it into a 6800 chip, you'd actually get lower performance than a 6600gt.(6600gt also has higher power consumption than 6800)