buy a celeron slot1 300a - 333 from FS/FT -- instant 12% - 25% overclock
depending on the motherboard you should be able to put in at least a 333 MHz, possibly even a (slot1 or slocket + ppga) 400-466 (all usually under $20 shipped in FS/FT).
also, depending on the motherboard it may not be overclockable, not all LX / ZX boards allowed any speeds above 66 MHz.