for 6870 memory underclocking you have to edit the bios: http://www.techpowerup.com/rbe/ (fully read and understand the tutorial!)
w7:
use latest afterburner beta: http://download1.msi.com/files/downloads/uti_exe/vga/MSIAfterburnerSetup220Beta.zip
and tweak configfile with:
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
google that, I don't remember exactly how to tweak it proper.
this worked and still works for my 2x msi r6870 twin frozr rig
Do at your own risk!
Reading this helped me a lot. Thank you!
The W7 method worked for my single 6870. GPU-Z & MSi both say I am running currently at 1021/525 [291 Mhash]. I still don't have access to the voltage, but that's okay. How far should I take my over/under-clocking?
EDIT: Without touching voltage, It seems that I max out at 1030/300 @ 70-75C Fan @ 50%