considering how old that pc is, the most substantial upgrade you can probably give it, is a currently available pata hdd, and most likely what would amount to a much faster pata hdd than whats currently in there, that would give a several fold perceived improvement compared to a slightly faster cpu. (and if the mb supports sata, all the better)
this will probably be the most worthwhile investment to make for it, seconded by a memory upgrade, most likely, regardless of the memory speed... because as long as you have enough to minimize pagefile usage, thats what matters most.
if youre concerned about being able to play games faster, consider a replacement gpu (and possibly a new psu too, depending on how demanding the gpu is)
as far as just upgrading the cpu by itself, it isnt worth it, not compared to anything else you could do, for possibly less money.