Knowing the model of the mobo would be useful...
Anyway, I think that your micro is this one:
http://processorfinder.intel.com/details.aspx?sSpec=SL4P2
It use the socket 370 and the memory type was SDRAM 66Mhz. The max memory size depends on the chipset of your mobo: ALi Aladdin TNT2, Intel 440BX, Intel 820E, Intel 820, SiS 630E, Intel 815E, Intel 815EP, VIA 694Z, VIA Pro133A, VIA 694XDP, VIA ProSavage PM133 or Intel 815E.
It's very difficult, although not impossible, to find such an old memory module. Kingston manufactures SDRAM 100Mhz modules which could be compatible.
Anyway, I think that you should look for it in the second-hand market.