The above recommendation is probably the best. I ran Debian on my p-120 with 48mb RAM (FreeBSD was snappier, though). I didn't use Slackware at that time, but its a fantastic distro for older equipment, and all the versions are still on mirrors for download. Gentoo would be slow to install because of the ampunt of RAM you have installed. If you get anything installed, I'd suggest you use IceWM, Windowmaker, or Fluxbox as your graphical environment.
Now to the nitty gritty. What I installed back then was Debian Woody, fresh out of the gates. When it was released (I downloaded it on the first day), it still defaulted to the 2.2 kernel, and had the 2.4-bf kernel available as an alternative. 2.2 was less memory intensive than the 2.4, so it ran faster, but 2.4 at the time had more support for newer hardware. The 2.2 kernel is still being maintained and patched (The latest is 2.2.26
as of today). I'd suggest that you consider using it, as it will fly on your machine compared to a 2.4 or 2.6 kernel.
That's my $0.02. Take it or leave it. BTW, at that time, KDE was at version 1.1 and it ran okay on that machine (it'd be like putting windows 98 on the one you have now - possible but slow).