Your video card is best overclocked by using Rivatuner or the nVidia registry tweak. Ask over in the video forum, or better yet, do a search here and on Google. Video card overclocking is easy.
Upping the AGP voltage will ultimately run a little more juice through your northbridge, and may wind up helping an extreme overclock, but it is something to try after you've gotten the idea of the limitations of your system. Max out your RAM and you chip first, and as you play around with dividers and FSB, then think about upping the AGP voltage.