Actually, with that old of a computer, you don't need to buy RAM - go to a junkyard or a dumpster. A late Pentium III or an early SDRAM Pentium IV should have the correct type of memory.
Also, if you find a working Pentium III-based system with a processor clocked at 1.2 GHz or higher, use it instead of your P4. Intel jacked up the efficiency big time when they introduced their new architecture.