If you want a distro of Linux I agree with MISRy.DSL is the distro to use. http://www.damnsmalllinux.org/
Also Puppy Linux too. http://puppylinux.org/wikka/HomePage
There are some other ones as well.
However If one wanted to retro game Windows 98,98SE would be best as those were the Operating Systems available around the era of the AMD K6-2 CPU.Windows ME would also run however many people don't like it as much as 98SE.
Linux is always the best operating system for any computer, of course. I used Linux way back when these machines were current, but you are probably better off using something that is designed for lightweight systems. At the very least, you will want to max out the RAM as much as you can.
For ease of use and installation, maybe try Linux Mint 5, XFCE edition. If that fails, I would look into DSL or Puppy, though I've never used those myself. There is definitely something that will work.