Well the latest bios for the BH6 is the SS bios and it says it supports up to 700 Mhz celeron. I still have the Abit BH6 with the QN bios installed running a celeron 566 overclocked to 850 with 1.85 volts, rock solid system. All you have to is go into the bios by pressing the delete key while the machine is started and changing the 66 bus speed to 100 and increasing the vcore to 1.85. Also the AGP ratio must be changed to 2/3 instead of 1/1. The other setting that says sell hi/low- 66/100 , don't seem to make any difference, I have it selected hi on my computer.
If you're interested, I want to upgrade this system and I would be willing to sell the cpu, heatsink, fan,2 sticks of 256k memory that can run at Cas 2 for $100, shipping included.