Damn Small Linux immediately comes to mind.
With sufficient creativity, you can even run
gentoo on old PC's... I've got it running on a Pentium 233 with a 1.2GB hard drive. I had to do crazy things like turn /usr/share/doc into a compressed filesystem, then mount it with unionfs... This is not reccomended if you lack insanity and obsessive compulsion.
The number one bottleneck with old PC's is going to be memory. There's no getting around a multitasking OS needing more memory than a single-tasking thing like DOS. Give it at
least 64 megs. Old mem is cheap and plentiful if you can find it.