Celeron 566 with a SlotKet, (if you get a lucky CPU) can do 850MHz on BH6.
The Celeron 566 is 8.5x66=566MHz. Switch the FSB to 100MHz, and you have an 850MHz system.
Log to
http://www.overclockers.com/
Click on CPU Database, Choose Celeron 566.
You will see few reports about BH6 @ 850MHz.
To choose a CPU like P-III ?700 and run it at 133FSB on BH6 is not a good idea, since the BH6 has only 1/3 divider to the pci. Which means that pci components (like HD) can have trouble running at 41Mhz.
The newer BX boards have ¼ divider for 133FSB so it is not a problem.