The IGP voltage is for the entire chipset.
I agree that running single channel and disabling the onboard graphics will give a better overclock. I'll be doing that one of these days as I'm waiting for Half Life2 to be released before I get an addin AGP card.
Upgrading the fan on the heatsink would be difficult IMO. The clearance between the fan and the bracket holding the heatsink down is only a few millimeteres so that would limit the size. I've been toying with the notion of adding in a different heatsink and found this one...TTIC-NPH-1
I'm not absolutely sure it will fit but it has the right design to work with the Ideq setup.
The stck cooler is actually pretty good. I can get my 2500+ up to 3200+ and a little beyond.
With the SmartFan utility you can manually adjust the speed of the fans. They get up to about 3000rpm and cool pretty well.
Get one and play around with it you'll like it.