If you have the SL series the most you can go up to is 400Mhz.
If you have the XL series I have successfully been able to use a K6-2/500 AFX CPU.
You can do this by Setting jumper J3 to 2.2 vcore
Set Jumper J13 (CPU/Host Ratio) to 5.0
Set Jumper J15 (CPU Clock Selection) to 100
For a 450Mhz CPU
You would make all the changes above except for J13 which should now be set to 4.5
I have tried to overclock the 500Mhz CPU and I've only been able to overclock it to 525Mhz without any issues.
At 550Mhz after awhile of use it eventually locks up.
At 575Mhz I get a message on the screen, but it doesn't boot.
Trying to get it to 600Mhz was impossible, the computer won't even boot.
I've used a Pentium III heatsink and fan to cool the CPU as well as using artic silver
One day I managed to find a 525Mhz CPU (without overclocking) and I have been successfully using it at 550Mhz.
Just curious as to what OS you are using?
I'm using Win98 but I've been dying to try and put Linux on the DiskonChip, but alas I'm linux deficient.
One of these days I'll bite the bullet.