No one seems to have 3Dmarked to compare a '6600GT AGP 500/1000mhz' with a '6800GS AGP 350/1000Mhz'.
No, and we may never see one done as the market loves to ignore AGP users.
However, I HAVE seen reviews comparing a 6600GT AGP (500mhz/8 pipes/128MB GDDR3)) vs a 6800NU AGP (325mhz/12 pipes/128MB DDR1).
When a 6600GT went up against the 6800NU, they essentially tied (give or take a few FPS) in most low-strenuous tests. However, when the AA was cranked up, the 6800NU always beat the 6600GT, thanks to the extra pipes... and that's regardless of the slower clock and the older memory type.
So it's clear the extra pipes are more important then core clock speed. Now the 6800GS is a lot like the 6800NU, except it has a slighty faster clock, improved memory type (GDDR3) and double the memory (256MB).
If the 6800NU could beat the 6600GT in most tests, then I can assure you, the 6800GS (stock) is a significantly better card then the 6600GT... now if you can unlock the extra pipes, then it's a HUGELY better card (I know, I went from a 6600GT to a 6800GT not long ago, the differance was significant).
So while we may not have an obvious review comparing the AGP 6600GT with the AGP 6800GS, I can assure you, the 6800GS is a good-deal faster then the 6600GT... even the 6800NU was faster.