Don't expect that much of an upgrade to be availible. The 440BX chipset only supports a maximum of 100MHz FSB. Pentium 3's soon went to the 133MHz FSB at around 600-700MHz time. Ontop of that the Slot one varietys are hard to come by now.
The best thing that you could perhaps get is an old 1.4GHz Celeron on the old SKT370 platform if its availible and get a convertor for it. The difference might not be that huge though and really, your better looking at getting a new mobo/CPU setup, if not a new machine.