To allow for further overclocking with Afterburner you have enable "Unofficial Overclocking" in the afterburner config file. Go into the MSI Afterburner folder in your Program Files (or wherever you installed it) and edit the "MSIAfterburner.cfg" file in notepad or any text editor.
Scroll down and near the bottom you need to make two changes.
Where it says "UnofficialOverclockingEULA =" add the following without the quotes "I confirm that I am aware of unofficial overclocking limitations and fully understand that MSI will not provide me any support on it"
Under that change "Enable Unofficial Overclocking" to "1". Restart Afterburner and you'll have access to a wider range of clock speeds.
Just be very careful, you can easily over/underclock way too high with it.
What do you think is safe for a 6950? and a 6990?