I'm getting 406mhash/s with my sapphire xtreme 5850 @ 975/300 no voltage change, operates around 68 degrees C
Do you have 1 card only?
I have 6 sapphire xtreme 5850 and the best card works stable @ 875/300 (the worst works @ 800/300), how did you made it work @ 975MHz ?
What OS do you use (Linux/Windows 32/64bits)?
What PSU do you use?