If the new ZM80c the same clearance as the old ZM80a, you can use taller heatsinks on the GPU side of the card. I use ThermalTake heat sinks--not spreaders--from this package: http://www.thermaltake.com/products/.../ddrCopper.htm
The only sinks I've found that fit under the non-GPU side of the of a ZM80a are these:
As a person who has overheated a 5800 Ultra to death with the stock cooler, I would not risk either of these 3rd party sinks on DDR2. Even the stock FX 5800 Ultra copper sometimes needs additional airflow, and the stock copper has far more surface area than these RAM sinks. If you go with 3rd party cooler on a 5800 Ultra, at least put a a fan blowing across the sinks.