If they are 486DX chips they should be 25, 33 or 40mHz. The 66mHz were the 486DX/2.they did for a time also make a 486/100/DX4. I have a friend who used a slew of them to tile a floor in his small bathroom. (Keychains are your best bet) I think you can at best load Win95, MAYBE 98, but other than a bit of slow web surfing those are not good for much.
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EDIT: AMD made some 486 clones(also called DX/4) There were even 5x133's too but there was NEVER a DX over 40mHz. anything higher was a DX/2 DX/4 etc...