You are correct in stating that the Abit
BH6 motherboard officially only supports up to a Pentium-III
500MHz CPU. To upgrade to an AMD Athlon,
you would require a new motherboard supporting Athlon's Slot-A connector
and a 200MHz front-side bus.
Checking our CPU Comparator, you would
see that an upgrade to the P3-500 would only net a performance increase of
about 10%, not likely worth the price of a new CPU.
Installing an Athlon-700 would almost double your performance (about an
80% increase in speed). Together with a new motherboard, the price these
days would start at around US$900 for the upgrade.