Although I have to agree with OtheHill; unless you're a nostalgia keeper like me or actually need to keep a legacy system around for important software, for as much money as a useful upgrade on that will cost you could get out and buy a reasonable Celeron laptop that would run 4x better as that.
As for the CPU, any CPU with socket 423 will most likely be compatible with it.
Also note that because that specific motherboard uses a special type of RAM (RDR, or Rambus Direct Ram or something, not too sure), you're gonna pay through the nose to get any significant amount of RAM for it. From what I read, RDR has to be installed in matched pairs and can't just be thrown haphazardly into the machine, so that means removing the sticks that are in there right now and buying all new RAM.
Not everyone can decipher Klingon script...
chay' ta' SoH tlhe' vam Doch Daq