Here are my current settings for the Sapphire 7950 Vapor-X, I have it all in a batch file I run when I need:

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer.exe --scrypt -o stratum+tcp://eu.wemineltc.com:3333 -u x -p x -I 20 -w 256 -g 1 --gpu-engine 900-1000 --gpu-memclock 1450 --gpu-fan 85 --auto-gpu --thread-concurrency 16384


This is getting me about 620KH/s, and haven't had a driver crash since setting it up.