It depends of which model of BH6 you have.
In any case you can use coppermine cored celeron or pentiumIII up to 1 Ghz as your CPU.
Go to
www.abit.nl / bios updates to see which cpu's the latest bios supports..
Upgrading CPU most probably requires at least a bios reflash.
The new tualatin core celeron or pentium3 won't work in bh6.
I'm running a celeron667@901 Mhz in the bh6 1.2...