I don't know if there is an equivalent setting in Ubuntu, but make sure the "Power Control" setting in AMD's Catalyst driver is at 20%. I have a single XFX 6970 running at 950 MHz and 420 Mhps with a temp that hovers around 70 degrees. If I set the "Power Control" to 0%, my speed drops when I go over 900 MHz.
That's a very good configuration.
I also have a XFX 6970. Clock @ 950, but my Power Control is at 5% getting 423Mhps. Runs about 70 degrees with the fan at 65%. I didn't see any increase when I moved the power control beyond 5%.