The OP asked about the AGP version of the 6600GT. In that case, both Arctic Silencers and Thermalright V1 will not fit due to the location of the GPU and the HSI chip. I would only suggest the Zalman VF700ALCU and leave the stock HSI bridge heatsink on.
I have the VF700CU and it is fine, it doesn't stress the card much at all, and cooling is phenomenal my idle dropped 8C and my load dropped a whopping 32C.