On a new powercolor from Newegg I'm getting around 500Mh/s. This after overclocking slightly in Catalyst to 1020Mhz (from stock 950Mhz). I haven't found an optimal config on cgminer to get those 600+ rates, right now I'm using this:
cgminer -o
http://coinotron.com:3334 -u username -p password --shaders 1792 --intensity 14 --worksize 256 -v 1
If I up the intensity the screen goes to hell and the system overall is unresponsive.
Ras