I like the 6600, it's a good solid mainstream product. But that's what it is only. There was that 6600LE OC version which you could unlock the pipelines and clock a bit more up and it would be in proximity of the 6600GT. There's the 6200 AGP (initial release version) which can be tuned to perform against a 6600 or better. And there's the 6800XT(?) which can also be clocked up to fight against the 6800 or near the 6800GT.
With a 6600, all you can do is clock it and hope to be somewhere in the middle of a vanilla and a 6600GT. Since there aren't any pipelines to unlock and clocking it also has its limits.
But it's a good product anyway, have one myself.
