There is probably a reason why you can't find the max voltage on the search engine.
Trying to overclock and increase voltage on a laptop is pretty risky stuff so I doubt many tried it.
What you can do is try the amd overclock tool and try to find safe OC ranges. First OC up by 20mhz, then another 10 at a time to find when your drivers shut themselfs off. You can probably game for hours using those safe overclocks. Don't overclock by 100mhz then another 100, your computer will just freeze up and may cause damage.
Asus G73jh-A1 ~ Windows 7 64 Bit ~ I7-720Qm ~ ATI 5870M (Sometimes OC to 780/1075) ~ 8 Gigs Ram ~ SSD