AFAIK, the 1/5 divider is all a myth, unless it has recently been made official. All that BIOS does is reduce PCI latencies, allowing for such higher overclocks like Insane3D. Meaning you'd be driving your PCI bus at 41Mhz when at 166Mhz and your AGP is at 83Mhz. Some configurations can handle this, some can't. Basically, it's the luck of the draw, if you can handle the upped FSB speeds, then you can hit 166. But the 1/5 divider isn't offical, at least not that I know of, so whether you're running 166 or 167Mhz you're still using the 1/4 divider for PCI and 2/4 for AGP.
--Mark