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.