It gave about a 20-30% real-world speedup.
You shouldn't have trouble replacing a Pentium 166 CPU with a Pentium 233MMX. $42 or so at [link|http://www.thechipmerchant.com|The Chip Merchant]. It wouldn't give a dramatic speed boost, but the larger on-chip cache will help and probably be noticable. I don't know anything first hand about faster chips with adapter sockets, etc., for Pentium systems.
HTH. Good luck.
Cheers,
Scott.