My guess is this works for all ATI cards (4XXX and up) but it may only work for certain cards only

I've tried countless test, and I've concluded that for my 6870 at least, I get up to 5% Mh/s increase by forcing idle clock for mem (300 from 1050). All tests I have done are with OCed core clock (1000~1045 from 900)

I used MSI afterburner. At 250 mem clock I got a 10% DECREASE, at 900~1200 mem clock I get a 5% DECREASE from mem clock at 300

Give it a try

http://event.msi.com/vga/afterburner/download.htm

Go in your MSI afterburner folder in program files (X86 for me) and edit the msiafterburner.cfg

where you see [ATIADLHAL] make sure it looks like 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

in order to get lower memory clocks please follow this:

1- Open MSI Afterburner
2- Set Memory Clock to the lowest possible or to desired amount
3- Click Apply
4- If Memory Clock is not to the desired amount, close MSI Afterburner
5- Repeat 1 to 4 until desired amount is reached
6- Set Fan Speed

Same can be done to get higher core clocks

Donations are always welcome 167q1CHgVjzLCwQwQvJ3tRMUCrjfqvSznd

EDIT: It seems the most effecient memory clock is between 300 and 400. In my case, for 1020 core on a 6870 it is 344, you should experiment as well