I believe that air-cooled heatsinks are more than sufficient, and there's no real benefit switching to water unless you're pumping chilled water over the memory.
Here's why.
Let's say we stick a small heatsink onto the memory chip and have a slow fan blowing over it. Let's assume that this small heatsink arrangement has a pretty bad C/W of 3.0. ie. for every 1W of power, the memory heats up by 3C.
Now memory chips by themselves won't suck down more than 0.5W in most circumstances, and perhaps 1W if they're RDRAM chips (which video cards don't use). Typically expect something like 0.25W per chip.
Let's assume a worst case scenario that the memory dissipates 1W. At a C/W of 3.0, the memory heats up by 3C above ambient.
So you want to water-cool the memory? Tell me, what is the typical rise above ambient for the water coolant in an overclocked computer system? If it's extremely good, it'll be 2-3C. Add in the thermal resistance of the memory block itself, and we're still looking at the memory climbing to ~3C above ambient.
What have we gained?
Just stick ramsinks on your card and have a slow blowing Papst running over it. You'll cool the memory just as well, or perhaps even better than, water.
As surprising as it may seem, water-cooling is overkill for certain scenarios, and memory is one of them.
