Lubuntu
http://lubuntu.net/ would be better than ubuntu, but the 3 Sirius suggests would be my top 3 for an old PC.
Slitaz as well may be worth a look.
I've got an old PII 450 I play with. Antix, DSL and Puppy and its offshoots run ok on it. Anything Ubuntu runs badly.
Debian, or Slackware can be made to run well, if you want to put the effort in yourself, otherwise Antix, DSL and Puppy are good 'cos other ppl have done the hard work for you
I generally end up going back to XP on that PC as it seems the best
