I got a nice boost when I switched from GUIMiner to bitminter miner. It works really well for 6000 series radeon cards. I was at 400s with GUIMiner and now I'm in 420s with 0.2% stale/rejected shares. The author is very active on this board and is constantly adding features that users want.
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.