I've used both the Zalman and the Tt Giant II. Both are very nice though the Giant II sorta blocks cooling to your Ram on the Video card.
BUT I've never encountered any REAL increases using Ram sinks etc. on any card I've owned. The ZALMAN is open allowing for air to fully cross the card. Both do the trick on the core very nicely. My Radeon 9800 NP 128mb has the Tt Giant II on it now. Runs excellent and my core increase has been tremendous.