These boards supports single voltage CPUs, as there are:

CPU-Upgrades with Adapter socket

On my Board, the maximum speed for the CPU was 5.0x66Mhz = 333MHz.
This was done, using a K6/2 and a K6/III.
I haven't tested a K6/2+ or K6/III+ yet.
Using higher speeds, the board didn't boot at all.

Tip:Before buying such a CPU-Upgrade get a L2-Cache module first