AGP setting are controled in the Bios on everyMB i have ever seen. there is no place to change the voltage in hte Bios you should have the option to chang from 2x 4x or 8x if the board and card support it. on my it is under the advanced options. but i have a Abit board.
EDIT: just thought about it your board may have a Jumper to change it.Abit uses a soft bios and has very few jumpers.