You'll want to be more specific about your question:
Performing a voltmod on the card by soldering a variable resistor onto the appropriate connection points can result in higher overclocks to whatever you modded.
Simply raising "AGP Voltage" in the BIOS has absolutely no effect on the overclockability of your video card. In some cases, raising AGP Voltage in the bios can help your front side bus speed of your system, as some machines link northbridge voltage to AGP voltage. But again, AGP Voltage does not change the overclocking abilities of your actual video card, it's memory, or it's GPU.