Originally Posted by Aphot
The 6800GS are actually 6800GT cores for agp. A select few unlock to 16 pipes and 6 vertex shaders. Im pretty sure the BFG one does and the XFX xxx edition one does too. The 7800GS isn't that much faster once you unlock the 6800GS.
Once you unlock the pipes and overclock it then it will be close, but once you add overclocking to the 7800GS it overtakes the 6800GS significantly. Plus the 7800GS has a better feature set.
As for the original question, if it has to be 6600GT or 6800GS, then 6800GS. If you have a couple extra bucks the 7800GS.