I have that same motherboard but mine has a K6-III+ 450 mhz CPU (0.18 micron process/2.0 V core) which runs much cooler than the standard K6 chips.
I am able to overclock it to 600 mhz without any special CPU cooler and just raising the voltage to 2.1 V.
The original K6-X chips are not good overclockers and their performance is also significantly slower than the K6+ vintage due to the difference in cache.
I bought mine from Tigerdirect for $49 in December, but I understand that they've sold out. The K6-2+ chips are availabe elsewhere for about the same price.
You can find out more info by tuning into the K6-X Series forum at: www.amdzone.com/forum