Cyrix have a special technology where their CPUs run at least 50% slower than other brands 
I used to happily use an AMD 5x86 @ 160Mhz on an ASUS VLB board, and "upgraded" to a Cyrix P150+ CPU - don't know why I bothered 
I think early 486 CPUs are mostly all the same though (licensed from Intel?), but as long as your Intel 486/66 is 5V, I'd put this in over the Cyrix. A little heatsink on top wouldn't go astray either.
JB
I used to happily use an AMD 5x86 @ 160Mhz on an ASUS VLB board, and "upgraded" to a Cyrix P150+ CPU - don't know why I bothered I think early 486 CPUs are mostly all the same though (licensed from Intel?), but as long as your Intel 486/66 is 5V, I'd put this in over the Cyrix. A little heatsink on top wouldn't go astray either.
JB