If you are only going to run 1080MHz on the memory, you don't need sinks at all. 1100 and down is fine with bare chips. Don't run memory sinks unless you have too, as you only have two choices for attaching them.
First is the thermal tape route, and this offers not only limited thermal conductivity but also a decent change of damaging your card in the act of pressing them on. The solder contacts on BGA ram are delicate, and essentially every failed card I've seen in the last 3 years was a simple case of failed memory solder joints. Most of these cards worked fine until people stuck memory sinks on them.
The other way (and right) way to go is to use a thermal epoxy and glue the memory sinks to the chips. This not only offers great thermal conductivity, but minimizes the chance of damaging the card (if you use a small amount of proper thermal adhesive like AC Alumina adhesive). The downside here is it voids the card's warranty in most cases.
As far as the core, 6600GTs do run pretty hot and better cooling is a fine idea even at stock clock. The Zalman will work well, but I would just keep the memory sinks for something that needs them, unless you are fine with epoxying them on. It simply isn't worth risking damaging the card in application (or removal) via the thermal tape route if 1080MHz is alright with you.
The AC VGA Silencer is not a bad option for the 6600GT. It will give you fairly effective memory cooling with miminal chances of damaging the card. And it will cool the core pretty darn well. VGA Silencers have trouble with the heat output of a 7800GT or GTX, but the 6600GT makes less heat and it pretty well within their limits.
