Ya, http://distrowatch.com/search.php might be a really good site to hit.
Put in "old computer" or similar, get as specific as you want and see what comes up.
I think you'll have a really tough time getting a modern mainstream distro to work on it. Most of them focus on computers that are being bought today, so they're concentrating on multiple 64-bit cores, large memory and that sort of thing. You don't have any of that.
Another option might be Gentoo, but you'll need to cross compile for it which would be much more entertaining. Scratch that one.