Use MSI afterburner and underclock the memory to something between 300 and 350MHz(find the best value for your card - each piece is different). I use the math GPUclock/3+14=MEMclock(found somewhere on this forum and for my card it is the best value)
To do so, you need to install afterburner, edit the file MSIafterburner.cfg by changing these lines to this:
[ATIADLHAL]
EnableUnofficialOverclocking = 1
UnofficialOverclockingEULA = I confirm that I am aware of unofficial overclocking limitations and fully understand that MSI will not provide me any support on it
UnofficialOverclockingMode = 2
AccessibilityCheckingPeriod = 0
Then run afterburner and underclock your memory to minimum possible(should be about 500MHz), shut down the afterburner and run it again. Now you should be able to underclock memory to desired 300 to 350MHz.
Memory underclock gave me about 15 degrees down, so I was able to overclock my 6870 to 1000MHz with stock cooler at 50% having GPU at 70'C