Weell, let's get started.
You wanna ride an Athlon? Here we're. We conducted tests among several mobos sporting AGP 4x, or otherwise said, with ths VIA KX133 chipset. Without any possibility of error, the only winner mainboard here is the new ABIT KA7, and her sister KA7 100, similar except for a second IDE controller embedded in the mainboard. And, it's an ATA100 controller, btw.
With normal coolers and the same ram, our Athlon 850 didn't manage to run faster than 875Mhz on an acclaimed Asus K7V. Many would say that it's because the CPU is near its limits. Well, with an KA7 and with a KA7 100, the same processor, with the same peripherals was able to run till 950MHz smoothly. That's a lot more!
Using an Athlon 750, that reached only 800MHz with the Asus MB, we were able to get to 860Mhz with the KA7!!  And then we tested an Athlon 700, which reached 830MHz.
I think that is owed to the six chips for ram stability control that Abit has and Asus, or any other common mobos hasn't.
Our Athlon 850 was able to get POST at 1020MHz!, and to get win98 loaded and then crash at 1003Mhz.
Now we're using some Globalwin Coolers, made up with large heatsink, three large fans, and a plastic air conduct to grab air from outside the cabinet. With it the A850 is running happily at 970MHz with 128mb made by HP with 6,2ns access time.

We can send you every piece of this overclocked system: the Athlon (and now Thunderbirds) CPUs, the Abit KA7 /100 mobos, and the GlobalWin Coolers model VOS32. They're rather expensive, 80$, but they're the best and suitable for SECC2 Intel CPUs too.

Table Of Results. ABIT KA7 ABIT KA7 + Global Win Cooler ASUS K7V
Athlon 850 950MHz 970MHz 875MHz
Athlon 750 860MHz 900MHz 800MHz
Athlon 700 830MHz n.t. 740MHz

This is our Athlon 850, on the Abit KA7, cooled by Global Win coolers VOS32+.