find out what your PLL is http://www.cpufsb.de/
click the "PLL find" link in the corner, and find what kind you have
then download the program, "CPUCool" from the same site, and follow its instructions
you may not have a supported PLL, but if you do, CPUCool will find it
(i didnt recommend Clockgen, because it only has around 12 PLL's, CPUCool has around 25)
It's an nForce 400 Ultra so you should have options in your BIOS. Check in the Advanced Chipset section for FSB and refernaces to CPU and RAM voltage.
If there greyed out. Look above them for "AUTO" and see if you have the option to change that to Manual, Expert or User. By changing the "Auto" setting the options should be available.