Just use MSIa, It has a backwards compatable "bug" in thier "unofficial overclocking" that allows you to Attempt to set Any clock to Anything
win7x64 Cat 12.1 SDK 2.6
crossfire6870 990core 490mem <---Sweet spot for mining
Hell, for awhile i had the cards running at 1020 core and 300mem because i was told 300mem was the best, It's the Most Common best, It's quite specific for the cards.
CGminer also changes clocks Flawlessly
I also hear (NEVER TOUCHED) that Shappire Trix is a good tool.
Doesn't seem to work for the 7970, though. The slider won't go below 685 memory clock.