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