I have 2 boards identical to this with removable cache, one now dead and one running a QFP cyrix 5x86 at 3x50mhz. Along with Feipoa, Pshipkov, BobocoCz I've done extensive testing of this one

These will not work at 66mhz fsb, even testing with CPU that run at that fsb on umc boards. 60mhz is doable, however you will have to turn off pci to dram buffer in bios at anything over 50mhz which in turn will severely limit the performance of any hdd connected via a pci bus hdd controller. Also turn off "byte merge" - it causes issues with some graphics cards. For high speeds with a cyrix I'd also recommend upgrading the voltage regulator. Also the stock bios is best for a Cyrix, am5x86 will not work properly at high multiplier with it, it will work best with the other available bioses (with which cache does not work properly with a cyrix).

This is a good board if you really like to rip out your hair in frustration (if you have any left). Good results can be had, but expect it to be picky about both sram and dram if you want results like sram at 3-1-1-1 and dram at faster/fastest at 50mhz and beyond. Especially with 1mb l2 things will be difficult, I could only ever get 512k working reliably at fast bus speeds