For lower memory clocks just select a different GPU and then switch back. It will let you set it lower. (this applies to both Afterburner and Trixx)
For me it doesn't. Don't know why. 3x5970s in Win7. I can switch back and forth, open, close, save, load, reboot, etc. It always limits it to 500. The most annoying thing is it seems to work for other people.
Driver version doesn't seem to matter, reinstall doesn't matter, nothing seems to matter.
The only thing I can think of it the fact that I have 2x 5970s from ATI and 1x5970 black edition from XFX. I know brand shouldn't matter but maybe something in the firmware in one or the other doesn't like the other.
Anyways I have found setting voltage in AB, and then using cgminer to set clocks works fine. I just wish cgminer could do everything and I would uninstall AB all together.
Trixx has been a complete POS for me. I won't even consider reinstalling it.