I know this is an old thread but I've managed to get my hands on 4 different models of 7950 and found that they vary widely in what settings they'll take even within the same maufacturer.  I track all settings in a spreadsheet: core, memory, intensity, MH/s (or KH/s for scrypt) and fan speed and temp.  Tune each card until you squeeze out that last bit of performance then drop each setting back a notch to keep the card from burning out.  The best results I've seen so far run from 580MH/s to 640MH/s.

Don't underestimate the importance of cooling, by the way.  In a generic case my cards ran 20C hotter than in a Coolermaster HAF with big fans. On a whim I put a small Vornado on top of a Coolermaster XB which dropped the two cards in there another 5C.

Windows and Linux seem to perform about the same with the same settings but Windows draws up to 20W more power when idle and 40 when doing housekeeping since it revs up the HDD and CPU.

If anyone's interested in my specific cards and settings let me know.  All of my benchmarking has been done by hand so far but I'm working on a script to do that for me.  PM or post if you want a copy once it's finished.   It'll be BASH so meant for Linux but it should work on cygwin and since it's using cgminer's API you could run it on Linux or Mac and point it at a Windows box.