Sure, I suppose for a reference board you can. In my experience, (with a non-reference 6950) the option to increase the voltage was only enabled in the vendor specific OC utility.
I have used afterburner as well.
One other question, do you know if Asus' warranty is transferable?
I tried to find info on transferability of the warranty, but couldn't. You can call them to find out:http://support.asus.com/Hotlines.aspx?SLanguage=en