I've been on a similar mission to yours before. Upgraded my bro's Pentium II 233Mhz to an Intel Celeron 466Mhz CPU, both 66Mhz bus...
I suggest you check out this Intel Motherboard Specification webpage: Intel RC440BX Motherboard Information
. You'll find what processors it can support in the Compatibility
This old PIII processor has a Slot 1 connector. Same applies for the PII.
You can get these slotket converters that convert a Slot 1 interface to a PPGA or FC-PGA interface, allowing you to use Celeron processors. You can get ones to convert Slot 1 for Pentium III processors.