I've just taken the plunge and installed Linux on my old decrepit PIII450.  I opted for Ubuntu as it seemed like a decent enough distro and someone gave me a CD!

I've been running it for a month or 2 and to be honest, I really like it!! 

The install was easy, everything worked 1st time and the Ubuntu forums are really helpful.

I've got samba working and I am now using the Linux box as a media server with Slimp3 server.  I have had no problems installing anything I have needed (think that more down to apt-get than anything else!) and it all seems to be really stable.

To be honest, other than playing games, I don't use my nice shiny AMD 3000+ XP machine for much any more.

So, if your thinking about Linux - Try it, you might like it!

Mark