Haven't tried putting ramsinks on my samsung 2.8 memory yet, but I have done the 1u copper cooler on the core. I only saw about 10MHz extra from that.
BTW, if you are getting the "copper" tweakmonster ramsinks, don't be fooled. They are not really copper. It's actually a copper alloy which looks more like brass to me. I bought a set and lapped the bottom, and they look nothing like copper.