In Trixx you have to disable ULPS (and possibly modify some registry keys).
In Afterburner, you have to edit the MSIAfterburner.cfg file using notepad and cut and paste a EULA sentence in there (unless you use the beta or if the beta has turned into a stable release):
http://forums.guru3d.com/showthread.php?t=338906That still doesn't allow you to go any lower.
From what I've seen of people able to go below 685, they're running more than one card. So just a guess, but us single card users might not be able to go any lower.