In the first case, one could easily run any modern linux on the box. In the second case, it is best to discourage the newbie. Her first contact with Linux would probably be a disaster and she might decide to never again try that crappy operating system.
I would suggest the following requirements:
- P200 for a reasonably usable desktop with X Windows, probably with a minimal distro like DSL or DeLi
- Hard drive: 600-800 MB for a bare minumum system, > 2 GB for a useful multi-purpose system, 3 GB for a full-sized system like Slackware
One can install and use Linux on hardware with lower specs. It might, however, be too difficult for a newbie to get a reasonable system running. For those willing to learn and use a simple system, even P100 will not be too old.