so an example - sapphire 7950 (reference):

auto-gpu is off
gpu-engine: 1100
gpu-memclock: 650  < OR > gpu-memdiff: -450 (i've tried both)
gpu-powertune: 10

but device monitor reports core clock 1100mhz, memory clock 1250mhz. Doesn't matter what values I put in the config, memory clock never seems to change. On my 7870s (tahiti LE), the memory clock defaults to 1500. Really need it lower...

If I use afterburner, I can manually set both clocks - but I've found some conflicts when both afterburner and cgminer are trying to modify clock.