In my own experience, and from what I've read on the net, it doesn't help your overclock. Does more harm than good sending unneccesary voltage to the agp bus...
Now, if you actually volt mod your video card to send more voltage to the gpu or the VRAM, then it definitely helps overclocking. Back in the day I had a 9800pro that wouldn't go past 420mhz on the core, and with a gpu vmod, I was hitting 450mhz easy.