Try Libranet. I have a K6-2 500 running Libranet and it works great with 256megs of RAM, max on Shuttle 567 motherboard. I set the multiplier to 2x, bus to 75mhz it runs at 6x75=450mhz. You won't run a lot of tasks at once, but it isn't bad. It was fun, now for $14!!
Why did I choose Libranet?
1) Redhat and other major distros have LOTS of things loaded to make it run on any PC. Libranet configures less, tailored to your old PC. The K6-2 was a slug with Fedora.
2) Easy to install and configure
3) GUI menu for system setup
4) Speed, all the unecessary items in #1 aren't loaded it runs faster
5) Several desktops, you choose, default is my favorite
6) Several browsers, you choose, I liked Opera, need to try Firefox on it some time
7) Clean, neat interface. ICEwm is great, well oraganized
8) You want more, Debian is under the hood, go for it
Yes, I even PAID for my copy. That's how impressed I was.