Alan D. Moore has written an excellent series on reviving an old PC with Linux.
Part I: Defining Expectations
Part II: Hardware Compatibility and Prep
Part III: Understanding Linux Distros
Part IV: Fully Lightweight Distros
Part V: The Remixes
Going through all of these will give you a pretty good idea of what you can expect trying to revive old hardware, and what you can do. He's done a lot better on this than I would ever have.
