The maximum is 14, but the higher you go, the less responsive your desktop will be and the higher the GPU temps will be, so be sure to turn up the fan speed.
I see by bumping it up to 5, you doubled your hash rate.
Hmm...I tried it briefly at I=14 and saw about 105 Mh/s but a lot of messages about rejected work because of delay.
The card at I=5 is running at around 130 degrees Fahrenheit according to the rule of finger, that is, me touching its center backside with my finger. Barely too hot to keep my finger on it. I suppose I'll have to install gkrellm and see what it's running at? Or is there an ATi command that will show its temperature?