As you could see, GPU voltage can be changed using some utilities. At present there are only two utilities capable of doing it — MSI Afterburner (mentioned earlier) and a new one, special for Fermi videcards created by German enthusiast Christian Roderfeld — NVIDIA Inspector. Each has its ups and downs, that is why I use them both in reference with my needs.
After several trials, I finally could reach 700 MHz without any voltage increase:
Excellent performance boost — 1600 point for videocard and 1700 for an overall score. GPU clocks are now the same as GTX 480 has, but GTX 470 performance would still yield to senior GF100 representative. Further overclocking using stock cooling system sounds useless because of high temperatures and noise level, therefore, the next step is to change it to waterblock.