There's a voltage and core/mem speed drop when in idle mode. But when running in performance (3D) mode it runs at it's normal voltage. The voltage will not increase any if you max out CCC or even go beyond it with Afterburner. The voltage will limit the absolute maximum overclock and to go higher you'll need to increase it manually. I ended up using ATI's RBE bios editor to set my 5850 permanently at 875/1200 with stock voltage and the great thing is it retains the lower 157/300 idle profile... compared to using Afterburner which will not allow it to return to idle state and also compared to CCC which maxes at 775/1125 speeds. And again, this is with stock voltage. In Afterburner I cranked it up to 985/1200 but this required a voltage increase from 1.08 to 1.22... so I don't use it. I want to keep it at stock volts.
Oh also I used RBE so that my CCC can go as high as 1000/1300 if I want... although I have no intention of actually doing that lol.