I posed basically the same question on the Zalman tech support site. I was told there is about 4mm of clearance between the heatsink, and PCB on one side (there is lot's of clearance on the other side). For my graphics card the tops of the memory on the PCB is already about 2mm above the PCB surface. The RAM sinks I would have used are at least 10mm high, and wouldn't work.
A possible solution might be: fit heat conducting paths to perfectly contact the heat sink and the memory on low clearance side of the PCB and then effectively be using the heatsink to cool the RAM on that side of the board.